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/tests/GpuDrawPathTest.cpp b/tests/GpuDrawPathTest.cpp
index 012dc61..79afa63 100644
--- a/tests/GpuDrawPathTest.cpp
+++ b/tests/GpuDrawPathTest.cpp
@@ -78,7 +78,7 @@
 
 DEF_GPUTEST_FOR_ALL_GL_CONTEXTS(GpuDrawPath, reporter, ctxInfo) {
     for (auto& test_func : { &test_drawPathEmpty, &test_drawSameRectOvals }) {
-        for (auto& sampleCount : {0, 4, 16}) {
+        for (auto& sampleCount : {1, 4, 16}) {
             SkImageInfo info = SkImageInfo::MakeN32Premul(255, 255);
             auto surface(
                 SkSurface::MakeRenderTarget(ctxInfo.grContext(), SkBudgeted::kNo, info,