blur: Invalidate newly-bound framebuffers before rendering

This signals to the GPU driver that the FBO contents do not need to be
preserved. According to ARM, invalidating framebuffers after rendering
and unbinding them won't do anything on Mali GPUs [1], but it improves
performance with Qualcomm's Adreno GPU drivers.

When tested with 2 layers of another 6-pass blur implementation, this
saves ~100 µs of rendering time on an Adreno 640 GPU at 1440x3040
resolution.

[1] https://community.arm.com/developer/tools-software/graphics/b/blog/posts/mali-performance-2-how-to-correctly-handle-framebuffers



Change-Id: Ib26e904e66fbc95924d14bbe132cf8a0505d4f19
Signed-off-by: Mohammad Hasan Keramat J <ikeramat@protonmail.com>
Signed-off-by: Simão Gomes Viana <devel@superboring.dev>