Decouple gl_hudscale and gl_consolescale
Implement gl_menuscale
Initialize the screen handling code before the renderer

This change is needed to break a otherwise fatal cycle:
- The renderer calls VID_MenuInit()
- VID_MenuInit() calls SCR_GetMenuScale()
- SCR_GetMenuScale() relies on gl_menuscale which is still
  uninitialized at this time.
Implement scaling of the menu

This commit has some drawbacks:
- It's rather hacky. The Quake II menu is crap and was never intended
  to be scaled. My approach was to add scaling to most of the generic
  functions and handle all the special cases in the non generic parts
  of the menu. A better solution would require to rewrite at least
  parts of the menu. And like it's said in qmenu.c: I won't do that.
- Some menu elements are aligned to the right, others to the left. In
  many places magic numbers are used to align elements by hand. This
  makes it very hard to impossible to implement a scaling logic which
  works in all situations. With this approach most menus look good up
  to at least a scaling factor of 3. Especially the "Player Setup"
  menu is very problematic at small disalignements are unavoidable.

Please note, that only the menu system itself is scaled. Some elements
like the the "Quit Screen" or the loading plaque are still missing. They
will be done in a later commit.
Implement scaling of several non-menu elements

- The loading plaque
- The pause plaque
- The "Quit Screen"
Document the 2D artwork scaling
Update the CHANGELOG