glamor: Avoid 2D bitblit if possible.
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 <email@example.com>