Fix flipped `interior` option for second refprobe

This patch fixes rendering of multiple reflection probes for a single
mesh in the GLES2 renderer.  If there were two reflection probes, one of
them would always have the `interior` option flipped, resulting in
broken blending between probes and flickering of ambient reflection.

Also make note of GLES2 reflection probe limit

doc/classes/ReflectionProbe.xml

@@ -7,6 +7,7 @@
 		Capture its surroundings as a dual paraboloid image, and stores versions of it with increasing levels of blur to simulate different material roughnesses.
 		The [ReflectionProbe] is used to create high-quality reflections at the cost of performance. It can be combined with [GIProbe]s and Screen Space Reflections to achieve high quality reflections. [ReflectionProbe]s render all objects within their [member cull_mask], so updating them can be quite expensive. It is best to update them once with the important static objects and then leave them.
 		[b]Note:[/b] By default Godot will only render 16 reflection probes. If you need more, increase the number of atlas subdivisions. This setting can be found in [member ProjectSettings.rendering/quality/reflections/atlas_subdiv].
+		[b]Note:[/b] The GLES2 backend will only display two reflection probes at the same time for a single mesh. If possible, split up large meshes that span over multiple reflection probes into smaller ones.
 	</description>
 	<tutorials>
 		<link>https://docs.godotengine.org/en/3.4/tutorials/3d/reflection_probes.html</link>

drivers/gles2/rasterizer_scene_gles2.cpp

@@ -2132,7 +2132,7 @@ void RasterizerSceneGLES2::_setup_refprobes(ReflectionProbeInstance *p_refprobe1
 		state.scene_shader.set_uniform(SceneShaderGLES2::REFPROBE2_USE_BOX_PROJECT, p_refprobe2->probe_ptr->box_projection);
 		state.scene_shader.set_uniform(SceneShaderGLES2::REFPROBE2_BOX_EXTENTS, p_refprobe2->probe_ptr->extents);
 		state.scene_shader.set_uniform(SceneShaderGLES2::REFPROBE2_BOX_OFFSET, p_refprobe2->probe_ptr->origin_offset);
-		state.scene_shader.set_uniform(SceneShaderGLES2::REFPROBE2_EXTERIOR, p_refprobe2->probe_ptr->interior);
+		state.scene_shader.set_uniform(SceneShaderGLES2::REFPROBE2_EXTERIOR, !p_refprobe2->probe_ptr->interior);
 		state.scene_shader.set_uniform(SceneShaderGLES2::REFPROBE2_INTENSITY, p_refprobe2->probe_ptr->intensity);
 
 		Color ambient;