Fix unit test crash caused by OSMesa not supporting rendering to GL_ALPHA8

src/gpu/gl/GrGLUtil.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "GrGLUtil.h"
 #include "SkMatrix.h"
@@ skipping 109 matching lines @@
         if (kNVIDIA_GrGLVendor == vendor) {
             *outDriver = kNVIDIA_GrGLDriver;
             int n = sscanf(versionString, "%d.%d.%d NVIDIA %d.%d",
                            &major, &minor, &rev, &driverMajor, &driverMinor);
             // Some older NVIDIA drivers don't report the driver version.
             if (5 == n) {
                 *outVersion = GR_GL_DRIVER_VER(driverMajor, driverMinor);
             }
             return;
         }
-
         int n = sscanf(versionString, "%d.%d Mesa %d.%d",
                        &major, &minor, &driverMajor, &driverMinor);
         if (4 == n) {
             *outDriver = kMesa_GrGLDriver;
             *outVersion = GR_GL_DRIVER_VER(driverMajor, driverMinor);
             return;
         }
     }
     else {
         if (kNVIDIA_GrGLVendor == vendor) {
@@ skipping 156 matching lines @@
         if (1 == n) {
             if (adrenoNumber >= 300) {
                 if (adrenoNumber < 400) {
                     return kAdreno3xx_GrGLRenderer;
                 }
                 if (adrenoNumber < 500) {
                     return kAdreno4xx_GrGLRenderer;
                 }
             }
         }
+        if (strcmp("Mesa Offscreen", rendererString)) {
+            return kOSMesa_GrGLRenderer;
+        }
     }
     return kOther_GrGLRenderer;
 }
 
 GrGLVersion GrGLGetVersion(const GrGLInterface* gl) {
     const GrGLubyte* v;
     GR_GL_CALL_RET(gl, v, GetString(GR_GL_VERSION));
     return GrGLGetVersionFromString((const char*) v);
 }
 
@@ skipping 32 matching lines @@
     GR_STATIC_ASSERT(2 == kGreater_StencilFunc);
     GR_STATIC_ASSERT(3 == kGEqual_StencilFunc);
     GR_STATIC_ASSERT(4 == kLess_StencilFunc);
     GR_STATIC_ASSERT(5 == kLEqual_StencilFunc);
     GR_STATIC_ASSERT(6 == kEqual_StencilFunc);
     GR_STATIC_ASSERT(7 == kNotEqual_StencilFunc);
     SkASSERT((unsigned) basicFunc < kBasicStencilFuncCnt);
 
     return gTable[basicFunc];
 }