Check Platform.glFoo in headless

[sprunk]

Platform.glHaveGLSL is defined to true, this is incorrect since headless doesn't actually have GLSL:

spring/rts/Lua/LuaConstPlatform.cpp

Line 96 in 34671c0

LuaPushNamedBool(L, "glHaveGLSL", true);

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?