Set roughness/metallic to 1 when assigning a texture in BaseMaterial3D

This makes material setup faster and avoids mistakes, especially with
the metallic channel which defaults to 0.

The value is only changed when adding a texture when none was
previously assigned, not when assigning a different texture.

doc/classes/BaseMaterial3D.xml

@@ -53,6 +53,7 @@
 			<argument index="1" name="texture" type="Texture2D" />
 			<description>
 				Sets the texture for the slot specified by [code]param[/code]. See [enum TextureParam] for available slots.
+				[b]Note:[/b] When setting a roughness or metallic texture on a material that has no texture assigned to those slots, [member roughness] or [member metallic] will automatically be set to [code]1.0[/code] to ensure correct appearance.
 			</description>
 		</method>
 	</methods>
@@ -229,6 +230,7 @@
 		</member>
 		<member name="metallic" type="float" setter="set_metallic" getter="get_metallic" default="0.0">
 			A high value makes the material appear more like a metal. Non-metals use their albedo as the diffuse color and add diffuse to the specular reflection. With non-metals, the reflection appears on top of the albedo color. Metals use their albedo as a multiplier to the specular reflection and set the diffuse color to black resulting in a tinted reflection. Materials work better when fully metal or fully non-metal, values between [code]0[/code] and [code]1[/code] should only be used for blending between metal and non-metal sections. To alter the amount of reflection use [member roughness].
+			[b]Note:[/b] [member metallic] is automatically set to [code]1.0[/code] when assigning a metallic texture using [method set_texture].
 		</member>
 		<member name="metallic_specular" type="float" setter="set_specular" getter="get_specular" default="0.5">
 			Sets the size of the specular lobe. The specular lobe is the bright spot that is reflected from light sources.
@@ -301,6 +303,7 @@
 		</member>
 		<member name="roughness" type="float" setter="set_roughness" getter="get_roughness" default="1.0">
 			Surface reflection. A value of [code]0[/code] represents a perfect mirror while a value of [code]1[/code] completely blurs the reflection. See also [member metallic].
+			[b]Note:[/b] [member roughness] is automatically set to [code]1.0[/code] when assigning a roughness texture using [method set_texture].
 		</member>
 		<member name="roughness_texture" type="Texture2D" setter="set_texture" getter="get_texture">
 			Texture used to control the roughness per-pixel. Multiplied by [member roughness].

scene/resources/material.cpp

@@ -1658,13 +1658,28 @@ bool BaseMaterial3D::get_feature(Feature p_feature) const {
 
 void BaseMaterial3D::set_texture(TextureParam p_param, const Ref<Texture2D> &p_texture) {
 	ERR_FAIL_INDEX(p_param, TEXTURE_MAX);
+
+	if (get_texture(TEXTURE_ROUGHNESS).is_null() && p_texture.is_valid() && p_param == TEXTURE_ROUGHNESS) {
+		// If no roughness texture is currently set, automatically set the recommended value
+		// for roughness when using a roughness map.
+		set_roughness(1.0);
+	}
+
+	if (get_texture(TEXTURE_METALLIC).is_null() && p_texture.is_valid() && p_param == TEXTURE_METALLIC) {
+		// If no metallic texture is currently set, automatically set the recommended value
+		// for metallic when using a metallic map.
+		set_metallic(1.0);
+	}
+
 	textures[p_param] = p_texture;
 	RID rid = p_texture.is_valid() ? p_texture->get_rid() : RID();
 	RS::get_singleton()->material_set_param(_get_material(), shader_names->texture_names[p_param], rid);
+
 	if (p_texture.is_valid() && p_param == TEXTURE_ALBEDO) {
 		RS::get_singleton()->material_set_param(_get_material(), shader_names->albedo_texture_size,
 				Vector2i(p_texture->get_width(), p_texture->get_height()));
 	}
+
 	notify_property_list_changed();
 	_queue_shader_change();
 }