In GLSL, a cube texture is sampled with one of textureCube* functions.
This function takes a 3D coordinate which is a vector from the center of
the cube and identifies a direction to sample from. GLES2.0 has the
following table that translates this 3D coordinate (Rx, Ry, Rz) to a
face and ST coordinates within that face. This table can be found in
Section 3.7.5 (Cube Map Texture Selection).

A compiler pass is implemented in ANGLE that replaces samplerCube
declarations with a sampler2DArray. The textureCube* functions are
replaced with the corresponding texture* functions with the translated
coordinates according to that table.

Gradients provided to textureCubeGrad are translated using the same
formulae, which is not precise but the spec specifies this projection to
be implementation dependent.

Helper invocations enabled through WQM (whole quad mode) cause a
nuisance in that the extrapolated varyings used as coordinates in a
textureCube call could have a different major axis (and therefore face)
from the non-helper invocations that lie within the geometry.
subgroupQuadSwap* operations are used in conjunction with
gl_HelperInvocation to make sure the helper threads calculate texture
UVs in the same face as the non-helper invocations.

Bug: angleproject:3300
Bug: angleproject:3240
Bug: angleproject:3243
Bug: angleproject:3732
Change-Id: I0cb6a9b1f2e1e6a392b5baca1c7118ed1c502ccf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1715977Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>