* You have to clean and invalidate the destination image so that
the processor writes out the full image and so that the processor
doesn't flush cache lines ontop of the GPU writing the image.
* The ICACHE doesn't automatically flush itself after a draw operation.
Depending on the situation, it will just hold the entire source
image internally and never read it again. In particular, when
scaling something like 160x120 it just caches the whole image
and never reads it again.