Merge branch 'sev/luma_hud_vehicles' into 'master'

Luma vehicle HUD revision

I intended to update some of the vehicle HUD images,
but ended up rewriting the vehicle HUD drawing code as well.

Changes from a developers perspective:
- The server sends more consistent vehicle stats, especially for secondary ammo.
- All vehicles now use a common HUD drawing function.
- All HUD images are drawn in relation to the frame image, rather than being measured individually.
- All vehicle crosshairs are scaled consistently, with cl_vehicles_crosshair_size.
- **Update**: The vehicle images are now fully integrated with the existing HUD themes.

Changes from a players perspective:
- Improved and more consistent crosshairs, also with a special healing crosshair for the Bumblebee.
- Secondary ammo is better represented. The bar shows the total ammo left, instead of reloading after each shot.
While the weapon is unusable during recharge, the icon is grayed out.
- Icons and bars are colored, using the health/armor colors from the main HUD for consistency.

![luma_vehicles](https://gitlab.com/xonotic/xonotic-data.pk3dir/uploads/832fdcf76cd3294494055b39cb3a85bb/luma_vehicles.png)

See merge request !148