Check `Platform.glFoo` in headless

`Platform.glHaveGLSL` is defined to `true`, this is incorrect since headless doesn't actually have GLSL:
https://github.com/beyond-all-reason/spring/blob/34671c0200e9be08ec2c51c6291353b760fbc69b/rts/Lua/LuaConstPlatform.cpp#L96

The task is to figure out which entries are incorrect this way, and fix them. I assume that in headless, all the `glHaveFoo` and `glSupportFoo` should be `false`, `availableVideoModes` should probably be empty, and `numDisplays` should probably be 0. Double-check this.

Stuff like `glVersion` probably stays as-is. Perhaps there could be a `Platform.isHeadless` or `Engine.isHeadless` as well?

Start in rts/Lua/LuaConstPlatform.cpp at the glHaveGLSL definition and inspect the related glHaveFoo, glSupportFoo, availableVideoModes, numDisplays, and version entries. Compare their headless behavior with the issue’s expectations, then correct values that are demonstrably wrong; done means headless reports no unavailable GLSL or video hardware while values such as glVersion remain appropriate.

Written by the indexing model from the issue text.