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

src/gpu/gl/GrGLContext.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef GrGLContext_DEFINED
 #define GrGLContext_DEFINED
@@ skipping 29 matching lines @@
      * Initializes a GrGLContextInfo from a GrGLInterface and the currently
      * bound OpenGL context accessible by the GrGLInterface.
      */
     bool initialize(const GrGLInterface* interface);
     bool isInitialized() const;
 
     GrGLBinding binding() const { return fBindingInUse; }
     GrGLVersion version() const { return fGLVersion; }
     GrGLSLGeneration glslGeneration() const { return fGLSLGeneration; }
     GrGLVendor vendor() const { return fVendor; }
+    /** Is this a mesa-based driver. Does not mean it is the osmesa software rasterizer. */
+    bool isMesa() const { return fIsMesa; }
     const GrGLCaps* caps() const { return fGLCaps.get(); }
     GrGLCaps* caps() { return fGLCaps; }
     const GrGLExtensions& extensions() const { return fExtensions; }
 
     /**
      * Shortcut for extensions().has(ext);
      */
     bool hasExtension(const char* ext) const {
         if (!this->isInitialized()) {
             return false;
         }
         return fExtensions.has(ext);
     }
 
     /**
      * Reset the information
      */
     void reset();
 
 private:
 
     GrGLBinding             fBindingInUse;
     GrGLVersion             fGLVersion;
     GrGLSLGeneration        fGLSLGeneration;
     GrGLVendor              fVendor;
     GrGLExtensions          fExtensions;
+    bool                    fIsMesa;
     SkAutoTUnref<GrGLCaps>  fGLCaps;
 };
 
 /**
  * Encapsulates the GrGLInterface used to make GL calls plus information
  * about the context (via GrGLContextInfo).
  */
 class GrGLContext {
 public:
     /**
@@ skipping 31 matching lines @@
     GrGLContextInfo& info() { return fInfo; }
 
 private:
     void reset();
 
     const GrGLInterface* fInterface;
     GrGLContextInfo      fInfo;
 };
 
 #endif