Fix glCopyTexSubImage2D inverted texture bug.

This bug only affects the software OpenGL ES
driver.
diff --git a/opengl/libagl/texture.cpp b/opengl/libagl/texture.cpp
index fa25fa9..9407bd5 100644
--- a/opengl/libagl/texture.cpp
+++ b/opengl/libagl/texture.cpp
@@ -1488,7 +1488,7 @@
         height = cbSurface.height - y;
 
     int err = copyPixels(c,
-            surface, xoffset, yoffset,
+            txSurface, xoffset, yoffset,
             cbSurface, x, y, width, height);
     if (err) {
         ogles_error(c, err);