pixman: update internal copy to pixman-0.32.6

commit a93a3af9 introduces use of PIXMAN_TYPE_RGBA, but it's only available
in pixman >= 0.21.8. Although commit f27b2e1d bumped pixman to pixman-0.28.2,
but the change was reverted later by 7b1b5d19.

This patch updates internal copy of pixman to pixman-0.32.6 to fix the
problem.
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

pixman @ 87eea99e

-Subproject commit 97336fad32acf802003855cd8bd6477fa49a12e3
+Subproject commit 87eea99e443b389c978cf37efc52788bf03a0ee0