Set correct uniform size for Metal.

There are two fixes here. The first pads out the uniform buffer to
match the alignment of the largest element in the buffer, as required
by Metal. The second ensures that the RTHeight uniform is only added
to the shader if required by the program inputs, as otherwise it wasn't
being included in the uniform data.

Bug: skia:8737
Change-Id: Icbe57456f3ec79faada231278c58137ca9865dd2
Reviewed-on: https://skia-review.googlesource.com/c/189875
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed