Add MESA detection to GrContextInfo and use to decide whether to use GL_ALPHA or GL_RED.

src/gpu/gl/GrGLUtil.cpp

Index: src/gpu/gl/GrGLUtil.cpp
diff --git a/src/gpu/gl/GrGLUtil.cpp b/src/gpu/gl/GrGLUtil.cpp
index 08bbebcbb04a967ab24fe39894e27c6cd7ef925c..a13b03de77577166f5e231b53e15bd08d15fe562 100644
--- a/src/gpu/gl/GrGLUtil.cpp
+++ b/src/gpu/gl/GrGLUtil.cpp
@@ -121,6 +121,12 @@ GrGLBinding GrGLGetBindingInUseFromString(const char* versionString) {
     return kNone_GrGLBinding;
 }
 
+bool GrGLIsMesaFromVersionString(const char* versionString) {
+    int major, minor, mesaMajor, mesaMinor;
+    int n = sscanf(versionString, "%d.%d Mesa %d.%d", &major, &minor, &mesaMajor, &mesaMinor);
+    return 4 == n;
+}
+
 GrGLVersion GrGLGetVersionFromString(const char* versionString) {
     if (NULL == versionString) {
         GrAssert(!"NULL GL version string.");