Dithering in OpenGL is vaguely defined, to the extent that no dithering
is also a valid dithering algorithm.  Dithering is enabled by default,
but emulating it has a non-negligible cost.  Similarly to some other
GLES drivers, ANGLE enables dithering only on low-bit formats where
visual banding is particularly common; namely RGBA4444, RGBA5551 and
RGB565.

Dithering is emulated in the fragment shader and is controlled by a spec
constant.  Every 2 bits of the spec constant correspond to one
attachment, with the value indicating:

- 00: No dithering
- 01: Dither for RGBA4444
- 10: Dither for RGBA5551
- 11: Dither for RGB565

The translator appends code to the shader that, based on the format
specified by the specialization constant, adds dithering to each color
attachment output.  A 2x2 Bayer matrix is used for dithering, indexed by
gl_FragCoord.xy % 2.

Bug: angleproject:6755
Change-Id: Ib45da5938e299b6626bff921119d63e7357dd353
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3374261


Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>