A draw that blends with the source alpha while setting the stencil (the
alpha on the PSP) needs dual source blending to write both. Without it the
alpha came out wrong, and later draws that blend with the destination alpha
differed: WipEout Pure, Juiced 2 and 16131 UCES00001 on a Mali-G71 and a
PowerVR GE8320. With framebuffer fetch, blend those in the shader instead,
which matches the Mac's Vulkan render now. Without fetch it would take a
framebuffer copy per draw, too slow for something this common.
A draw that writes no color can't need the source alpha, so it writes the
stencil as alpha directly.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
