For each internal shader, there is now possibly a companion .json file
that defines shader variations.  The variations consist of a set of
flags and a set of enums.  Each enum defines a variation that takes one
of a few values.  Flags are shorthands for 2-value enums.

In the shader code, #if can be used to change the shader based on
variations derived from flags and enums.  On the C++ side, those
variations are combined into an index and the appropriate shader is
retrieved from a table.

Bug: angleproject:2958
Change-Id: Ic4fc7061adb20c047c26624305285e3005092aab
Reviewed-on: https://chromium-review.googlesource.com/c/1351117
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>