In GrSkSLFP, advance the iterator pointer while building the key
In GrSkSLFP, advance the iterator pointer while building the key

We were checking the flags for each uniform, but never advancing the
uniform iterator. As a result, for each uniform that was specialized,
we'd just fold in another copy of the *first* uniform's data.

This led to a nasty performance regression (extra shader compiles) in
certain scenarios. (And it was a correctness issue, as well).

Change-Id: Iaac878a7b9eb37e98815f56dc55a58d1f8e85c22
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/420820
Reviewed-by: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia