Fixes rendering bugs and nondeterminism in gm.
Before, mixed samples stencil clip would try to infer whether the draw
wanted co-centered sample locations from within GrGLGpu, which caused
various errors. This change reworks it so the draw itself can request
the co-centered sample locations when it knows it will need them.
Also reduces framebuffer binds by moving the code that enables
GL_FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS into flushRenderTarget.
Review URL: https://codereview.chromium.org/1431593006
14184d5567