The PVRTC format, as pointed out by Le Hoang Quyen and Geoff Lang,
uses 4x4 or 8x4 blocks, but due to sampling from adjacent blocks,
requires a minimum size of 2x2 blocks per the OpenGL extension:
https://www.khronos.org/registry/OpenGL/extensions/IMG/IMG_texture_compression_pvrtc.txt
. Thanks to Quyen and Geoff for pointing out the intricacies.

Add a helper function to formatutils.cpp which returns the minimum
number of blocks (width and height) for a given compressed texture
format, and incorporate this into the compressed texture size
computation.

This patch makes WebKit on ANGLE pass WebGL's PVRTC compressed texture
test on iOS hardware.

Bug: angleproject:4652
No-Presubmit: True
Change-Id: I1046a091321b7948d712d16686ee0cb8795b8c99
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2213676
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>