gfx_sys.h provides functions and macros related to low level graphics system functions

• SPRITE_FLIP_NONE - Apply no sprite flip
• SPRITE_FLIP_X - Apply horizontal sprite flipping
• SPRITE_FLIP_Y - Apply vertical sprite flipping
• SPRITE_FLIP_BOTH - Apply sprite flipping on both axes

• QUADRANT_0 - Upper left quadrant of Sprite RAM page
• QUADRANT_1 - Upper right quadrant of Sprite RAM page
• QUADRANT_2 - Bottom left quadrant of Sprite RAM page
• QUADRANT_3 - Bottom right quadrant of Sprite RAM page

void init_graphics();

Call this to initialize the graphics system and prepare it for sequential drawing. Should only need to be called once at the beginning of the program.

void await_vsync(int frames);

Call this to block execution until the next vsync (frames=1) or wait for a number of vsyncs to pass (frames>1). Best used at the end of drawing and updating the game state.

void flip_pages();

Switch which framebuffer is being drawn on and which framebuffer is being sent to the TV. Call this right after await_vsync for consistent timing.

void await_drawing();

Waits for the current draw operation to complete.

Graphics API calls that use the blitter will set the global `draw_busy` variable to 1, and this function waits for the interrupt to set it to zero.