Danylo Piliaiev authored Feb 20, 2019 and Francisco Jerez committed Mar 25, 2019

From "Alpha Coverage" section of SKL PRM Volume 7:
 "If Pixel Shader outputs oMask, AlphaToCoverage is disabled in
  hardware, regardless of the state setting for this feature."

From OpenGL spec 4.6, "15.2 Shader Execution":
 "The built-in integer array gl_SampleMask can be used to change
 the sample coverage for a fragment from within the shader."

From OpenGL spec 4.6, "17.3.1 Alpha To Coverage":
 "If SAMPLE_ALPHA_TO_COVERAGE is enabled, a temporary coverage value
  is generated where each bit is determined by the alpha value at the
  corresponding sample location. The temporary coverage value is then
  ANDed with the fragment coverage value to generate a new fragment
  coverage value."

Similar wording could be found in Vulkan spec 1.1.100
"25.6. Multisample Coverage"

Thus we need to compute alpha to coverage dithering manually in shader
and replace sample mask store with the bitwise-AND of sample mask and
alpha to coverage dithering.

The following formula is used to compute final sample mask:
  m = int(16.0 * clamp(src0_alpha, 0.0, 1.0))
  dither_mask = 0x1111 * ((0xfea80 >> (m & ~3)) & 0xf) |
     0x0808 * (m & 2) | 0x0100 * (m & 1)
  sample_mask = sample_mask & dither_mask
Credits to Francisco Jerez <currojerez@riseup.net> for creating it.

It gives a number of ones proportional to the alpha for 2, 4, 8 or 16
least significant bits of the result.

GEN6 hardware does not have issue with simultaneous usage of sample mask
and alpha to coverage however due to the wrong sending order of oMask
and src0_alpha it is still affected by it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109743



Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>