Use MODELVIEW_MATRIX for vertex transform when double precision is enabled

This commit updates the double precision vertex transform code from
using the MODEL_MATRIX to now use the MODELVIEW_MATRIX instead.

This can be made possible by transforming the MODELVIEW_MATRIX
back into model space (ie, same space as the MODEL_MATRIX) and then using it as if it were the MODEL_MATRIX.

With this in place we now properly handle VERTEX transformations that
a Material Shader might do, such as billboard-ing.

(cherry picked from commit 63c6f0b93c)

servers/rendering/renderer_rd/shaders/forward_clustered/scene_forward_clustered.glsl

@@ -360,7 +360,7 @@ void vertex_shader(in uint instance_index, in bool is_multimesh, in uint multime
 		vertex = mat3(matrix) * vertex;
 		model_origin = double_add_vec3(model_origin, model_precision, matrix[3].xyz, vec3(0.0), model_precision);
 	}
-	vertex = mat3(model_matrix) * vertex;
+	vertex = mat3(inv_view_matrix * modelview) * vertex;
 	vec3 temp_precision; // Will be ignored.
 	vertex += double_add_vec3(model_origin, model_precision, scene_data.inv_view_matrix[3].xyz, view_precision, temp_precision);
 	vertex = mat3(scene_data.view_matrix) * vertex;

servers/rendering/renderer_rd/shaders/forward_mobile/scene_forward_mobile.glsl

@@ -347,8 +347,6 @@ void main() {
 #CODE : VERTEX
 	}
 
-	/* output */
-
 // using local coordinates (default)
 #if !defined(SKIP_TRANSFORM_USED) && !defined(VERTEX_WORLD_COORDS_USED)
 
@@ -361,7 +359,7 @@ void main() {
 		vertex = mat3(matrix) * vertex;
 		model_origin = double_add_vec3(model_origin, model_precision, matrix[3].xyz, vec3(0.0), model_precision);
 	}
-	vertex = mat3(model_matrix) * vertex;
+	vertex = mat3(inv_view_matrix * modelview) * vertex;
 	vec3 temp_precision;
 	vertex += double_add_vec3(model_origin, model_precision, scene_data.inv_view_matrix[3].xyz, view_precision, temp_precision);
 	vertex = mat3(scene_data.view_matrix) * vertex;