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====== gfx_sys.h ======

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

===== Defines =====
==== Sprite flips ====
  * 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

==== Sprite RAM Sections ====
  * 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

===== Functions =====
==== init_graphics ====
<code C>
void init_graphics();
</code>
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.
==== await_vsync ====
<code C>
void await_vsync(int frames);
</code>
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.
==== flip_pages ====
<code C>
void flip_pages();
</code>
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.
==== await_drawing ====
<code C>
void await_drawing();
</code>
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.