clarify that there's no kMedium here

Reword to make it clearer we're only handling kNone, kLow, and kHigh;
kMedium has been transformed into kLow or kHigh already at this point.

I think it's particularly easy to be confused by the line

    if (quality > kLow_SkFilterQuality) { ... }

Change-Id: If78cb6e946e26d08f5acd807d32e0446c69061b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261765
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

Author: Mike Klein

src/shaders/SkImageShader.cpp

@@ -499,7 +499,7 @@ bool SkImageShader::doStages(const SkStageRec& rec, SkImageStageUpdater* updater
         bool src_is_normalized = SkColorTypeIsNormalized(info.colorType());
 
         // Bicubic filtering naturally produces out of range values on both sides of [0,1].
-        if (quality > kLow_SkFilterQuality) {
+        if (quality == kHigh_SkFilterQuality) {
             p->append(SkRasterPipeline::clamp_0);
             p->append(fClampAsIfUnpremul ? SkRasterPipeline::clamp_1
                                          : SkRasterPipeline::clamp_a);
@@ -597,6 +597,7 @@ bool SkImageShader::doStages(const SkStageRec& rec, SkImageStageUpdater* updater
         p->append(SkRasterPipeline::move_dst_src);
 
     } else {
+        SkASSERT(quality == kHigh_SkFilterQuality);
         p->append(SkRasterPipeline::save_xy, sampler);
 
         sample(SkRasterPipeline::bicubic_n3x, SkRasterPipeline::bicubic_n3y);