Vulkan_Instance contains variables related to persistent vulkan resources like instance, device, queue, swapchain, etc.
Vulkan_Instance is re-iniitalized only during window creation.
Vulkan_Resources contains data specific to current game context, i.e. current level.
Vulkan_Resources can be re-loaded during level change, level exit, etc.
This optimization is not important nowdays. Also fast pass was disabled by default in original code.
This change simplifies rendering code since now we have only two shader stage processing functions: generic and sky.
r_renderAPI specifies rendering API to use (0 - opengl, 1 - vulkan).
r_renderAPICompareWindow shows additional window which does the same rendering as the main windows but uses different graphics API.
The idea is to use r_renderAPICompareWindow for debugging to be sure that both APIs render identical/similar pictures.
Removed r_colorbits, use desktop color depth as default value. Nevertheless if there are issues when looking
for corresponding pixel format it is still posible to obtain pixel format with color depth that is different
from desktop color.
No changes to game modules, which means that game UI can still set r_colorbits but it has no effect.
