GPUBatch: GL backend isolation

This changes the drawing paradigm a bit. The VAO configuration is done
JIT-style and depends on context active shader.

This is to allow more flexibility for implementations to do optimization
at lower level.

The vao cache is now its own class to isolate the concept. It is this
class that is reference by the GLContext for ownership of the containing
VAO ids.

source/blender/gpu/GPU_shader_interface.h

@@ -80,7 +80,7 @@ typedef struct GPUShaderInterface {
   /** Buffer containing all inputs names separated by '\0'. */
   char *name_buffer;
   /** Reference to GPUBatches using this interface */
-  struct GPUBatch **batches;
+  void **batches;
   uint batches_len;
   /** Input counts. */
   uint attribute_len;
@@ -109,8 +109,8 @@ const GPUShaderInput *GPU_shaderinterface_ubo(const GPUShaderInterface *, const
 const GPUShaderInput *GPU_shaderinterface_attr(const GPUShaderInterface *, const char *name);
 
 /* keep track of batches using this interface */
-void GPU_shaderinterface_add_batch_ref(GPUShaderInterface *, struct GPUBatch *);
-void GPU_shaderinterface_remove_batch_ref(GPUShaderInterface *, struct GPUBatch *);
+void GPU_shaderinterface_add_batch_ref(GPUShaderInterface *interface, void *cache);
+void GPU_shaderinterface_remove_batch_ref(GPUShaderInterface *interface, void *cache);
 
 #ifdef __cplusplus
 }