gl: optimization for gaussian blur

1. make scratch surface as single sample to avoid blitting between
multisample renderbuffer and texture
2. make a y-axis pass using scratch texture
gl: use separate set of scratch surface for both source and mask blur
gl/msaa:  force to use msaa compositor is app specifies it using a flag

This useful for shadow/blur where single sample texture is desirable via
msaa compositor without blitting