svga: implement 'white_fragments' option for VGPU10 fragment shaders
svga: implement 'white_fragments' option for VGPU10 fragment shaders

When we emulate XOR logicop mode with blend-subtract, we need to ensure
that the fragment shader always emits white.  We had this implemented
for VGPU9, but not VGPU10.

VMware bug 1545492.

Reviewed-by: Charmaine Lee <67ce69700661ea856134ddb10419a330e5f037d7@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler