Translator: Handle GS and TES output gl_Position like VS The spec declares these special shader variables identically as: ``` out gl_PerVertex { highp vec4 gl_Position; }; ``` Between vertex, geometry and tessellation evaluation shaders. Since the output is nameless, vertex shaders have always treated `gl_Position` as if it's a variable on its own. In this change, TES and GS treat that variable the same way. Additionally, this change removes `gl_Position` as a standalone variable from tessellation _control_ shaders, which must always use `gl_out` instead. Bug: angleproject:349994211 Change-Id: Ib33592fb62af31021a99f76f77c27da214425826 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/7042770 Auto-Submit: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>