Redefine the meaning of sample counts in GPU backend.

Old: 0  -> nonMSAA
     1+ -> MSAA

New:
     0  -> error/unsupported
     1  -> nonMSAA
     2+ -> MSAA

We still allow 0 to mean nonMSAA in three sets of public APIs for backwards compatibility:

1) SkSurface factories
2) GrBackendRenderTarget constructors
3) GrCaps::getSampleCnt()'s requestedCount parameter

However, we immediately clamp to 1 and treat 0 as invalid/non-renderable internally.

This also changes the behavior when using a large sample count. We now fail in that case rather than using the largest sample available sample count. GrCaps::getSampleCount() will return 0 in this case.


Bug: skia:
Change-Id: Ida22c6b22c1365e563c9046b611e88bf5eb3ff33
Reviewed-on: https://skia-review.googlesource.com/101560
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/GrBlurUtils.cpp b/src/gpu/GrBlurUtils.cpp
index b79a38a..b6e3092 100644
--- a/src/gpu/GrBlurUtils.cpp
+++ b/src/gpu/GrBlurUtils.cpp
@@ -110,7 +110,7 @@
                                              int sampleCnt) {
     if (GrAA::kNo == aa) {
         // Don't need MSAA if mask isn't AA
-        sampleCnt = 0;
+        sampleCnt = 1;
     }
 
     sk_sp<GrRenderTargetContext> rtContext(context->makeDeferredRenderTargetContextWithFallback(