Add cap and builder feature for multisample interpolation

src/gpu/glsl/GrGLSLCaps.cpp

Index: src/gpu/glsl/GrGLSLCaps.cpp
diff --git a/src/gpu/glsl/GrGLSLCaps.cpp b/src/gpu/glsl/GrGLSLCaps.cpp
index 0aa3e4ffeac5f3faed509a52d0464ff5e72f95d5..3b6c205d4b515ba3003b790419baea63727ae55f 100755
--- a/src/gpu/glsl/GrGLSLCaps.cpp
+++ b/src/gpu/glsl/GrGLSLCaps.cpp
@@ -25,6 +25,7 @@ GrGLSLCaps::GrGLSLCaps(const GrContextOptions& options) {
     fMustForceNegatedAtanParamToFloat = false;
     fFlatInterpolationSupport = false;
     fNoPerspectiveInterpolationSupport = false;
+    fMultisampleInterpolationSupport = false;
     fSampleVariablesSupport = false;
     fSampleMaskOverrideCoverageSupport = false;
     fVersionDeclString = nullptr;
@@ -33,6 +34,7 @@ GrGLSLCaps::GrGLSLCaps(const GrContextOptions& options) {
     fSecondaryOutputExtensionString = nullptr;
     fExternalTextureExtensionString = nullptr;
     fNoPerspectiveInterpolationExtensionString = nullptr;
+    fMultisampleInterpolationExtensionString = nullptr;
     fSampleVariablesExtensionString = nullptr;
     fFBFetchColorName = nullptr;
     fFBFetchExtensionString = nullptr;
@@ -67,6 +69,8 @@ SkString GrGLSLCaps::dump() const {
     r.appendf("Flat interpolation support: %s\n", (fFlatInterpolationSupport ?  "YES" : "NO"));
     r.appendf("No perspective interpolation support: %s\n", (fNoPerspectiveInterpolationSupport ?
                                                              "YES" : "NO"));
+    r.appendf("Multisample interpolation support: %s\n", (fMultisampleInterpolationSupport ?

[joshualitt, 2016/03/01 17:58:58]

Should we log the string? we don't do it for fb fetch support, but we do seem to
do it for kAdvBlendEqInteractionStr

[bsalomon, 2016/03/01 18:02:26]

Seems like it'd be nice to log both.

[joshualitt, 2016/03/01 18:02:45]

whoops, should have paid more attention, the string is either a nullptr or
"GL_OES_shader_multisample_interpolation", nvm

+                                                          "YES" : "NO"));
     r.appendf("Sample variables support: %s\n", (fSampleVariablesSupport ? "YES" : "NO"));
     r.appendf("Sample mask override coverage support: %s\n", (fSampleMaskOverrideCoverageSupport ?
                                                               "YES" : "NO"));