Win32: Ask for destination alpha in the OpenGL context. Fixes some issues in non-buffered rendering, see #3436.

This apparently still happens on Android so will need to fix it separately there.
Merge branch 'master' of https://github.com/hrydgard/ppsspp.git into unicode_cn
Merge branch 'unicode_cn' of https://github.com/kaienfr/ppsspp_cn into unicode_cn