Use GL_ALPHA instead of GL_RED to create texture in mesa if no GL_ARB_texture_rg extension

src/gpu/gl/GrGLCaps.cpp

Index: src/gpu/gl/GrGLCaps.cpp
===================================================================
--- src/gpu/gl/GrGLCaps.cpp	(revision 9423)
+++ src/gpu/gl/GrGLCaps.cpp	(working copy)
@@ -161,10 +161,18 @@
                          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) {
+        const GrGLubyte* verUByte;
+        GR_GL_CALL_RET(gli, verUByte, GetString(GR_GL_VERSION));
+        const char* ver = reinterpret_cast<const char*>(verUByte);
+        if (NULL != strstr(ver, "Mesa")) {
+            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");
     }