the context flag is immutable, and it's set by drivers that always
require the shader to export a psiz value for rasterization
"always" is all cases except GL_VERTEX_PROGRAM_POINT_SIZE being enabled, in which case it should be assumed that the shader already writes it, and no changes should be made
thus the shader key value can be changed to reflect that, when set, it requires the shader to export a psiz value if it doesn't already
this enables the pass to be run during precompile