For the componentAlpha with PictOpOver, we use two pass
rendering to implement it. Previous implementation call
two times the glamor_composite_... independently which is
very inefficient. Now we change the control flow, and do
the two pass internally and avoid duplicate works.
For the x11perf -rgb10text, this optimization can get about
30% improvement.
Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>