Fix merge request of branch 'gfx-api-rework' into 'master'

All changes of !22 (merged) are not present in 'master' and 'gfx-api-rework' is gone now. So merge 'em again.

See merge request !22 (merged)

(cherry picked from commit fc08eb23)

34b22d1c Engine: Add a separate method for asserting current graphical API.
9ba14d2a Engine: Utilize GetCurrentAPI() and CheckAPI() graphical methods.
3164eab8 Engine: Add method for getting graphical API name by its type.
763e191b Engine: Rewrite CGfxLibrary::HasAPI() to be able to check any API.
1f91fd3b Engine: Move GfxAPIType enum from GfxLibrary.h to Gfx_wrapper.h.
7c213d7b Engine: Add header file for graphical function interface.
b801f2b2 Engine: Add header files for all currently supported graphical APIs.
d2de43b9 Engine: Add method for setting new graphical API by its type.
f650e632 Engine: Replace graphical function pointers with proper interfaces.