gpu,gpu/internal: drop implied transformation from BlitFramebuffer

The Metal (and presumably the D3D11) backend doesn't support transformed
framebuffer blits. The only caller doesn't need it either, so drop that
capability from the driver abstraction.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/internal/driver/driver.go

@@ -45,7 +45,7 @@ type Device interface {
 	BindFragmentUniforms(buf Buffer)
 	BindStorageBuffer(binding int, buf Buffer)
 
-	BlitFramebuffer(dst, src Framebuffer, srect, drect image.Rectangle)
+	BlitFramebuffer(dst, src Framebuffer, srcRect image.Rectangle, dstOrigin image.Point)
 	MemoryBarrier()
 	DispatchCompute(x, y, z int)