[Fixed] Shader compilation problems on Vulkan

[Graf Zahl]

Reporting because right now I cannot address this myself:

A user shader using 'texture2D' to look up a texture fails to compile with Vulkan. Manually changing 'texture2D' to 'texture' makes it work.
Discovered with Freaky Panties.

[_mental_]

The problem exists with Apple’s OpenGL too. But with Vulkan it affects all platforms.
I would solve this by adding texture?D functions to our shader header. This solution was already used for noise functions.

[Graf Zahl]

I don't think that's a good idea - on Windows 'texrture2D' is still a valid builtin function. So this has to be done platform specific or the text has to be replaced in-place before compilation.

[_mental_]

Our shader header is different for OpenGL and Vulkan.
I mentioned Apple’s OpenGL just for the sake of completeness. I have no plans to fix it.
With Vulkan renderer texture2D needs to be handled regardless of platform.

[Graf Zahl]

It also needs to get fixed on OpenGL, because if Apple fails here, there's no guarantee that other implementors also remove deprecated functionality.

[Graf Zahl]

I tried to fix it as a function override, but that doesn't work. It seems 'texture2d' is a token, not an identifier, so the only way to change it is to patch the shader source.