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

src/gpu/gl/GrGLUtil.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLUtil_DEFINED
 #define GrGLUtil_DEFINED
 
@@ skipping 53 matching lines @@
 ////////////////////////////////////////////////////////////////////////////////
 
 /**
  * Helpers for glGetString()
  */
 
 // these variants assume caller already has a string from glGetString()
 GrGLVersion GrGLGetVersionFromString(const char* versionString);
 GrGLBinding GrGLGetBindingInUseFromString(const char* versionString);
 GrGLSLVersion GrGLGetGLSLVersionFromString(const char* versionString);
+bool GrGLIsMesaFromVersionString(const char* versionString);
 GrGLVendor GrGLGetVendorFromString(const char* vendorString);
 
 // these variants call glGetString()
 GrGLBinding GrGLGetBindingInUse(const GrGLInterface*);
 GrGLVersion GrGLGetVersion(const GrGLInterface*);
 GrGLSLVersion GrGLGetGLSLVersion(const GrGLInterface*);
 GrGLVendor GrGLGetVendor(const GrGLInterface*);
 
 /**
  * Helpers for glGetError()
@@ skipping 68 matching lines @@
     do {                                                        \
         GR_GL_CALLBACK_IMPL(IFACE);                             \
         (RET) = (IFACE)->f##X;                                  \
         GR_GL_LOG_CALLS_IMPL(X);                                \
     } while (false)
 
 // call glGetError without doing a redundant error check or logging.
 #define GR_GL_GET_ERROR(IFACE) (IFACE)->fGetError()
 
 #endif