Implement cockpit view - Githubissues

Fixes cockpit camera direction and position
Fixes skybox model and rendering used in cockpit mode
Updates BR_MODU_* enum names from BRender 1.3 that seem to match better than the BRender 1.2 SDK names
Added support for rendering with BR_RSTYLE_EDGES - useful for debugging
Reverted to using the colour_buffer width/height/offset when reading the depth buffer which resolves 1 and 2 from #282

dethrace-labs / dethrace