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It turns out that the use of fbo blit is one of the root cause
which lead to slow drawing, especially slow filling rects.

We guess there should be a performance bug in the mesa driver
or even in the kernel drm driver. Currently, the only thing
glamor can do is to avoid calling those functions.

We check whether the copy source and destination has overlapped
region, if it has, we have to call fbo blit function. If it has
not, we can load the source texture directly and draw it to the
target texture. We totally don't need the glCopyPixels here, so
remove it.

By apply this patch, the rendering time of firefox-planet-gnome
decrease to 10.4 seconds. At the same platform, uxa driver get 13
seconds. This is the first time we get better performance than
uxa driver.
Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>