b=567272; wglMakeCurrent is expensive; r=bas

gfx/thebes/src/GLContextProviderWGL.cpp

@@ -207,8 +207,17 @@ public:
 
     PRBool MakeCurrent()
     {
-        BOOL succeeded = sWGLLibrary.fMakeCurrent(mDC, mContext);
-        NS_ASSERTION(succeeded, "Failed to make GL context current!");
+        BOOL succeeded = PR_TRUE;
+
+        // wglGetCurrentContext seems to just pull the HGLRC out
+        // of its TLS slot, so no need to do our own tls slot.
+        // You would think that wglMakeCurrent would avoid doing
+        // work if mContext was already current, but not so much..
+        if (sWGLLibrary.fGetCurrentContext() != mContext) {
+            succeeded = sWGLLibrary.fMakeCurrent(mDC, mContext);
+            NS_ASSERTION(succeeded, "Failed to make GL context current!");
+        }
+
         return succeeded;
     }