Specialization constant are used not just for flip/rotation. It also
used for other things. This CL merges all specialization constant usage
(lineRasterEmulation, flip, rotation, halfRenderArea) into one class and
rename FlipRotationSpecConst to SpecConst.

Bug: b/173800146
Change-Id: I8dc3354b6caedbb183cec29855fc1c301ec8872a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2555812
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>