Implement global and per instance shader uniforms.

Adds two keywords to shader language for uniforms:
-'global'
-'instance'

This allows them to reference values outside the material.

servers/rendering/rasterizer_rd/shaders/canvas_uniforms_inc.glsl

@@ -132,6 +132,11 @@ layout(set = 2, binding = 6) uniform sampler shadow_sampler;
 
 #endif
 
+layout(set = 2, binding = 7, std430) restrict readonly buffer GlobalVariableData {
+	vec4 data[];
+}
+global_variables;
+
 /* SET3: Render Target Data */
 
 #ifdef SCREEN_TEXTURE_USED

servers/rendering/rasterizer_rd/shaders/scene_high_end_inc.glsl

@@ -134,7 +134,7 @@ struct InstanceData {
 	mat4 transform;
 	mat4 normal_transform;
 	uint flags;
-	uint instance_ofs; //instance_offset in instancing/skeleton buffer
+	uint instance_uniforms_ofs; //base offset in global buffer for instance variables
 	uint gi_offset; //GI information when using lightmapping (VCT or lightmap)
 	uint layer_mask;
 };
@@ -287,6 +287,11 @@ cluster_data;
 
 layout(set = 0, binding = 15) uniform texture2D directional_shadow_atlas;
 
+layout(set = 0, binding = 16, std430) restrict readonly buffer GlobalVariableData {
+	vec4 data[];
+}
+global_variables;
+
 // decal atlas
 
 /* Set 1, Radiance */

servers/rendering/rasterizer_rd/shaders/sky.glsl

@@ -58,6 +58,11 @@ params;
 
 layout(set = 0, binding = 0) uniform sampler material_samplers[12];
 
+layout(set = 0, binding = 1, std430) restrict readonly buffer GlobalVariableData {
+	vec4 data[];
+}
+global_variables;
+
 #ifdef USE_MATERIAL_UNIFORMS
 layout(set = 1, binding = 0, std140) uniform MaterialUniforms{
 	/* clang-format off */