GPUFramebuffer: Add recursive downsampling function.

This special case function enables rendering to a miplevel while using the miplevels above as texture input.
This is needed for some algorithm (i.e. creating a min-max depth pyramid texture).

source/blender/gpu/GPU_framebuffer.h

@@ -75,6 +75,10 @@ void GPU_framebuffer_blit(
         GPUFrameBuffer *fb_read, int read_slot,
         GPUFrameBuffer *fb_write, int write_slot, bool use_depth);
 
+void GPU_framebuffer_recursive_downsample(
+        GPUFrameBuffer *fb, struct GPUTexture *tex, int num_iter,
+        void (*callback)(void *userData, int level), void *userData);
+
 /* GPU OffScreen
  * - wrapper around framebuffer and texture for simple offscreen drawing
  * - changes size if graphics card can't support it */