Externals / VideoCommon: update glslang to latest, turn into submodule

[iwubcode]

To prepare for another change, I needed to update glslang. This updates us to the latest tag 15.4.0 and moves us to a submodule.

Previously, our build system for both CMake/Visual Studio would specify all the source files and effectively attempt to specify the right build files. This was cumbersome during an update, needing to determine which includes or source files were still around or which had been removed.

glslang also has another issue (which may be why we weren't using a submodule), this is that the project has a template file. Instead of attempting to create this file appropriately, we were just hardcoding the file.

Looking around, I noticed RPCS3 actually invokes the glslang's CMake build system inside the Visual Studio project. This allows us to completely remove the specific build files and fixes the template file issue.

The implementation (in Dolphin) is a bit klunky and there is currently some duplication in settings but the build seemed to work on my system. It does have an additional copy step to copy the *.lib files built by CMake to the output directory for use in the build. Also, I couldn't find a better way to include the necessary libraries, so like FFMPEG and some of our other dependencies, there are additional libs included in our base props.

Finally, the glslang update had some other changes:

• The TIntermediate class functions aren't public any more, instead they've moved to the TShader
• DefaultTBuiltInResource is being removed from the public interface in the next release, figured we should prepare for this.