Implement stencil clipping in mixed sampled render targets

This change enables multisampled clipping for mixed sampled
render targets.  Previously clipping in mixed samples config
behaved the same as in the gpu config.

In order to retrofit non-MSAA draw methods, programmable sample
locations are used in order to colocate all samples at (0.5, 0.5).
Requires support for NV_sample_locations.

BUG=skia:4399

Review URL: https://codereview.chromium.org/1232103002
13 files changed