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

src/gpu/gl/GrGLCaps.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "GrGLCaps.h"
 #include "GrGLContext.h"
@@ skipping 146 matching lines @@
 
     fTextureUsageSupport = (kES2_GrGLBinding == binding) &&
                             ctxInfo.hasExtension("GL_ANGLE_texture_usage");
 
     // Tex storage is in desktop 4.2 and can be an extension to desktop or ES.
     fTexStorageSupport = (kDesktop_GrGLBinding == binding &&
                           version >= GR_GL_VER(4,2)) ||
                          ctxInfo.hasExtension("GL_ARB_texture_storage") ||
                          ctxInfo.hasExtension("GL_EXT_texture_storage");
 
-    // ARB_texture_rg is part of OpenGL 3.0
+    // ARB_texture_rg is part of OpenGL 3.0, but mesa doesn't support it if
+    // it doesn't have ARB_texture_rg extension.
     if (kDesktop_GrGLBinding == binding) {
-        fTextureRedSupport = version >= GR_GL_VER(3,0) ||
-                             ctxInfo.hasExtension("GL_ARB_texture_rg");
+        if (ctxInfo.isMesa()) {
+            fTextureRedSupport = ctxInfo.hasExtension("GL_ARB_texture_rg");
+        } else {
+            fTextureRedSupport = version >= GR_GL_VER(3,0) ||
+                                 ctxInfo.hasExtension("GL_ARB_texture_rg");
+        }
     } else {
         fTextureRedSupport = ctxInfo.hasExtension("GL_EXT_texture_rg");
     }
 
     fImagingSupport = kDesktop_GrGLBinding == binding &&
                       ctxInfo.hasExtension("GL_ARB_imaging");
 
     // ES 2 only guarantees RGBA/uchar + one other format/type combo for
     // ReadPixels. The other format has to checked at run-time since it
     // can change based on which render target is bound
@@ skipping 394 matching lines @@
     GrPrintf("GL_ARB_imaging support: %s\n", (fImagingSupport ? "YES": "NO"));
     GrPrintf("Two Format Limit: %s\n", (fTwoFormatLimit ? "YES": "NO"));
     GrPrintf("Fragment coord conventions support: %s\n",
              (fFragCoordsConventionSupport ? "YES": "NO"));
     GrPrintf("Vertex array object support: %s\n", (fVertexArrayObjectSupport ? "YES": "NO"));
     GrPrintf("Use non-VBO for dynamic data: %s\n",
              (fUseNonVBOVertexAndIndexDynamicData ? "YES" : "NO"));
     GrPrintf("Core Profile: %s\n", (fIsCoreProfile ? "YES" : "NO"));
     GrPrintf("Discard FrameBuffer support: %s\n", (fDiscardFBSupport ? "YES" : "NO"));
 }