Fix mixed samples stencil clip

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
15 files changed