Reduce gets of ext string and check validity of stencil format enums

Review URL: http://codereview.appspot.com/4678043/



git-svn-id: http://skia.googlecode.com/svn/trunk@1801 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/src/GrGpuGLShaders.cpp b/gpu/src/GrGpuGLShaders.cpp
index b474be2..13f7543 100644
--- a/gpu/src/GrGpuGLShaders.cpp
+++ b/gpu/src/GrGpuGLShaders.cpp
@@ -248,14 +248,12 @@
 GrGpuGLShaders::GrGpuGLShaders() {
 
     resetContext();
-    int major, minor;
-    gl_version(&major, &minor);
 
     f4X4DownsampleFilterSupport = true;
     if (GR_GL_SUPPORT_DESKTOP) {
-        fDualSourceBlendingSupport =
-            major > 3 ||(3 == major && 3 <= minor) ||
-            has_gl_extension("GL_ARB_blend_func_extended");
+        fDualSourceBlendingSupport = 
+                            fGLVersion >= 3.3f ||
+                            this->hasExtension("GL_ARB_blend_func_extended");
     } else {
         fDualSourceBlendingSupport = false;
     }