An initial implementation based on AMD's FFX SPD (Single Pass
Downsampler).

Apart from requiring STORAGE_IMAGE support for the image format, the
following limitations are in place due to FFX SPD:

- Image must be 2D or 2D array (including cube maps)
- Image must be single-sampled

The following _can_ be supported, but not yet implemented:

- sRGB formats
- Integer formats
- depth/stencil formats

Bug: angleproject:4551
Change-Id: Ibc4d5cea701cca31e55e3d651540872bbd3b473f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2278713Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>