Implement vertex shading

This adds support in all backends, but the Compatibility renderer works the best.
Mobile and Forward+ can only support one directional light shader (the first in the tree)
While the Compatibility renderer supports any number of shadows.

Co-authored-by: Clay John <claynjohn@gmail.com>

Author: 2 people

doc/classes/ProjectSettings.xml

@@ -2840,11 +2840,6 @@
 		</member>
 		<member name="rendering/shading/overrides/force_vertex_shading" type="bool" setter="" getter="" default="false">
 			If [code]true[/code], forces vertex shading for all rendering. This can increase performance a lot, but also reduces quality immensely. Can be used to optimize performance on low-end mobile devices.
-			[b]Note:[/b] This setting currently has no effect, as vertex shading is not implemented yet.
-		</member>
-		<member name="rendering/shading/overrides/force_vertex_shading.mobile" type="bool" setter="" getter="" default="true">
-			Lower-end override for [member rendering/shading/overrides/force_vertex_shading] on mobile devices, due to performance concerns or driver support.
-			[b]Note:[/b] This setting currently has no effect, as vertex shading is not implemented yet.
 		</member>
 		<member name="rendering/textures/canvas_textures/default_texture_filter" type="int" setter="" getter="" default="1">
 			The default texture filtering mode to use for [CanvasItem]s built-in texture. In shaders, this texture is accessed as [code]TEXTURE[/code].

drivers/gles3/rasterizer_scene_gles3.cpp

@@ -4131,6 +4131,9 @@ RasterizerSceneGLES3::RasterizerSceneGLES3() {
 		global_defines += "\n#define MAX_DIRECTIONAL_LIGHT_DATA_STRUCTS " + itos(MAX_DIRECTIONAL_LIGHTS) + "\n";
 		global_defines += "\n#define MAX_FORWARD_LIGHTS " + itos(config->max_lights_per_object) + "u\n";
 		global_defines += "\n#define MAX_ROUGHNESS_LOD " + itos(sky_globals.roughness_layers - 1) + ".0\n";
+		if (config->force_vertex_shading) {
+			global_defines += "\n#define USE_VERTEX_LIGHTING\n";
+		}
 		material_storage->shaders.scene_shader.initialize(global_defines);
 		scene_globals.shader_default_version = material_storage->shaders.scene_shader.version_create();
 		material_storage->shaders.scene_shader.version_bind_shader(scene_globals.shader_default_version, SceneShaderGLES3::MODE_COLOR);