Expands existing struct-sampler rewrite to flatten arrays of arrays.
This allows us to support dynamically-uniform array indexing, which is
core in ES 3.2.

Samplers inside (possibly nested) structs are broken apart as before,
and then if the type resulting from merging the array sizes of the field
and its containing structs is an array of array, the array is flattened.

Also adds an offset parameter to functions taking in arrays to account
for this translation.

As a result of outer array sizes leaking into function signatures,
functions taking arrays of different sizes are duplicated according to
how the function is invoked.

Bug: angleproject:3604
Change-Id: Ic9373fd12a38f19bd811eac92e281055a63c1901
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1744177
Commit-Queue: James Dong <dongja@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>