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

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 a8c5425c6490916914fbd13286f85cf8bcd10079..46b58f1d768bf3f1aaeb3f95f51a0e27c48f8b35 100644
--- a/src/gpu/gl/GrGLUtil.cpp
+++ b/src/gpu/gl/GrGLUtil.cpp
@@ -127,7 +127,6 @@ void GrGLGetDriverInfo(GrGLStandard standard,
             }
             return;
         }
-
         int n = sscanf(versionString, "%d.%d Mesa %d.%d",
                        &major, &minor, &driverMajor, &driverMinor);
         if (4 == n) {
@@ -304,6 +303,9 @@ GrGLRenderer GrGLGetRendererFromString(const char* rendererString) {
                 }
             }
         }
+        if (strcmp("Mesa Offscreen", rendererString)) {
+            return kOSMesa_GrGLRenderer;
+        }
     }
     return kOther_GrGLRenderer;
 }