bf_gpu: Add GPU_state module.

This has wrappers for the most common gl* functions in the codebase, and is in preparation for D3502

Reviewers: brecht, fclem

Differential Revision: https://developer.blender.org/D3501

source/blender/gpu/GPU_framebuffer.h

@@ -194,6 +194,9 @@ void GPU_offscreen_viewport_data_get(
         GPUOffScreen *ofs,
         GPUFrameBuffer **r_fb, struct GPUTexture **r_color, struct GPUTexture **r_depth);
 
+void GPU_clear_color(float red, float green, float blue, float alpha);
+void GPU_clear(GPUFrameBufferBits flags);
+
 #ifdef __cplusplus
 }
 #endif