gfx: Move SDL initialization to gfx_init(), gfx_deinit()
gfx: Add higher-level sprite drawing functions

Add sprite drawing functions for all sprite
types in gfx, and use these functions throughout
the source code.
gfx: Wrap underlying SDL rectangle drawing functions

Use gfx_draw_rect() and gfx_fill_rect() throughout
the source code.
gfx: Add frame drawing functions
gfx: Add screen resolution functions
sdl-video: Remove unused masked sprite cache parameter

This was previously used to locally cache masked sprites in viewport. This
is no longer used since the introduction of viewport landscape prerendering.
Merge branch 'gfx-wrap-sdl'

Merge changes that wraps SDL rendering and screen functions in gfx.c. After
this change only gfx.c and freeserf.cc make calls to SDL.

This is based on the gfx functions proposed by Wdigger in #161.