pixman: update internal copy to pixman-0.32.6

commit a93a3af introduces use of PIXMAN_TYPE_RGBA, but it's only available
in pixman >= 0.21.8. Although commit f27b2e1 bumped pixman to pixman-0.28.2,
but the change was reverted later by 7b1b5d1.

This patch updates internal copy of pixman to pixman-0.32.6 to fix the
problem.

Signed-off-by: Hu Tao <[email protected]>
Signed-off-by: Gerd Hoffmann <[email protected]>

Author: Hu Tao