This cl introduces per sample shading in the fragment shader.
Rather than call the fragment shader multiple times per sample,
this cl adds a potential loop in the fragment shader where each
sample is processes in one of the loop's iteration.

- Each multisample related loop now processes either all samples,
  like before, or the current sample, if per sample shading is
  enabled
- A new per sample PixelProgram::maskAny() function was added
- emitEpilog() now has an option not to clear phis in order to be
  able to only clear them on the last sample
- The routine's fragCoord values are set per sample, with the
  proper sample offsets
- Similarly, the xxxx and yyyy values used for interpolation are
  now offset with the proper sample offsets when per sample
  shading is enabled

Bug: b/171415086
Change-Id: Ibd0c1bad23e2d81f7fa97240ebb50f88f1fee36e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51733Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>