Fix link error when using varyings with "dx_".

Varyings beginning with "dx_" would not get decorated properly, or
at all, which could cause potential internal variables and certainly
caused link errors. Fix this by no longer treating the "dx_" prefix
differently.

Also fix our naming of "dx_Position" to be consistent with our other
internal types.

BUG=angle:678

Change-Id: I03da0494a3d934d82ae7b3f8f12a576ff9bc3869
Reviewed-on: https://chromium-review.googlesource.com/203777
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libGLESv2/renderer/d3d11/Renderer11.cpp b/src/libGLESv2/renderer/d3d11/Renderer11.cpp
index a90b2cd..bd54252 100644
--- a/src/libGLESv2/renderer/d3d11/Renderer11.cpp
+++ b/src/libGLESv2/renderer/d3d11/Renderer11.cpp
@@ -1944,7 +1944,7 @@
 
 unsigned int Renderer11::getReservedVaryings() const
 {
-    // We potentially reserve varyings for gl_Position, _dx_Position, gl_FragCoord and gl_PointSize
+    // We potentially reserve varyings for gl_Position, dx_Position, gl_FragCoord and gl_PointSize
     return 4;
 }