Use negative mipmap LOD bias for sub-native bilinear 3D scale

This provides a benefit similar to FSR 1.0 (greater texture sharpness
at the cost of some graininess at sub-native resolution scales), but
without the added performance cost of FSR 1.0.

doc/classes/ProjectSettings.xml

@@ -1974,7 +1974,7 @@
 		</member>
 		<member name="rendering/textures/default_filters/texture_mipmap_bias" type="float" setter="" getter="" default="0.0">
 			Affects the final texture sharpness by reading from a lower or higher mipmap (also called "texture LOD bias"). Negative values make mipmapped textures sharper but grainier when viewed at a distance, while positive values make mipmapped textures blurrier (even when up close). To get sharper textures at a distance without introducing too much graininess, set this between [code]-0.75[/code] and [code]0.0[/code]. Enabling temporal antialiasing ([member rendering/anti_aliasing/quality/use_taa]) can help reduce the graininess visible when using negative mipmap bias.
-			[b]Note:[/b] When the 3D scaling mode is set to FSR 1.0, this value is used to adjust the automatic mipmap bias which is calculated internally based on the scale factor. The formula for this is [code]-log2(1.0 / scale) + mipmap_bias[/code].
+			[b]Note:[/b] If [member rendering/scaling_3d/scale] is lower than [code]1.0[/code] (exclusive), [member rendering/textures/default_filters/texture_mipmap_bias] is used to adjust the automatic mipmap bias which is calculated internally based on the scale factor. The formula for this is [code]log2(scaling_3d_scale) + mipmap_bias[/code].
 			[b]Note:[/b] This property is only read when the project starts. To change the mipmap LOD bias at run-time, set [member Viewport.texture_mipmap_bias] instead.
 		</member>
 		<member name="rendering/textures/default_filters/use_nearest_mipmap_filter" type="bool" setter="" getter="" default="false">

doc/classes/Viewport.xml

@@ -267,7 +267,7 @@
 		</member>
 		<member name="texture_mipmap_bias" type="float" setter="set_texture_mipmap_bias" getter="get_texture_mipmap_bias" default="0.0">
 			Affects the final texture sharpness by reading from a lower or higher mipmap (also called "texture LOD bias"). Negative values make mipmapped textures sharper but grainier when viewed at a distance, while positive values make mipmapped textures blurrier (even when up close). To get sharper textures at a distance without introducing too much graininess, set this between [code]-0.75[/code] and [code]0.0[/code]. Enabling temporal antialiasing ([member ProjectSettings.rendering/anti_aliasing/quality/use_taa]) can help reduce the graininess visible when using negative mipmap bias.
-			[b]Note:[/b] When the 3D scaling mode is set to FSR 1.0, this value is used to adjust the automatic mipmap bias which is calculated internally based on the scale factor. The formula for this is [code]-log2(1.0 / scale) + mipmap_bias[/code].
+			[b]Note:[/b] If [member scaling_3d_scale] is lower than [code]1.0[/code] (exclusive), [member texture_mipmap_bias] is used to adjust the automatic mipmap bias which is calculated internally based on the scale factor. The formula for this is [code]log2(scaling_3d_scale) + mipmap_bias[/code].
 			To control this property on the root viewport, set the [member ProjectSettings.rendering/textures/default_filters/texture_mipmap_bias] project setting.
 		</member>
 		<member name="transparent_bg" type="bool" setter="set_transparent_background" getter="has_transparent_background" default="false">

servers/rendering/renderer_rd/renderer_scene_render_rd.cpp

@@ -2459,8 +2459,7 @@ void RendererSceneRenderRD::render_buffers_configure(RID p_render_buffers, RID p
 		p_internal_width = p_width;
 	}
 
-	const float texture_mipmap_bias = -log2f(p_width / p_internal_width) + p_texture_mipmap_bias;
-	material_storage->sampler_rd_configure_custom(texture_mipmap_bias);
+	material_storage->sampler_rd_configure_custom(p_texture_mipmap_bias);
 	update_uniform_sets();
 
 	RenderBuffers *rb = render_buffers_owner.get_or_null(p_render_buffers);

servers/rendering/renderer_viewport.cpp

@@ -138,7 +138,11 @@ void RendererViewport::_configure_3d_render_buffers(Viewport *p_viewport) {
 
 			p_viewport->internal_size = Size2(render_width, render_height);
 
-			RSG::scene->render_buffers_configure(p_viewport->render_buffers, p_viewport->render_target, render_width, render_height, width, height, p_viewport->fsr_sharpness, p_viewport->texture_mipmap_bias, p_viewport->msaa, p_viewport->screen_space_aa, p_viewport->use_taa, p_viewport->use_debanding, p_viewport->get_view_count());
+			// At resolution scales lower than 1.0, use negative texture mipmap bias
+			// to compensate for the loss of sharpness.
+			const float texture_mipmap_bias = log2f(MIN(scaling_3d_scale, 1.0)) + p_viewport->texture_mipmap_bias;
+
+			RSG::scene->render_buffers_configure(p_viewport->render_buffers, p_viewport->render_target, render_width, render_height, width, height, p_viewport->fsr_sharpness, texture_mipmap_bias, p_viewport->msaa, p_viewport->screen_space_aa, p_viewport->use_taa, p_viewport->use_debanding, p_viewport->get_view_count());
 		}
 	}
 }