Add test for runtime effects that don't need sample coords
Add test for runtime effects that don't need sample coords

As explained in the bug and comment - if the only reference to sample
coords is from a sample call that gets converted to pass-through, we
don't *actually* need the coords. We don't realize that with today's
analysis, so we end up generating a set of coords for the FP, which is
wasteful (adds an extra varying).

Bug: skia:11869
Change-Id: Iff818bb2c3deb96162a8cf107ccf4ecb35344bb1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/411244
Reviewed-by: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Reviewed-by: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

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