/**
 * Basic sprite compiler
 * 
 * GTE has some specific needs that makes existing tools (like MrSprite) inappropriate. GTE
 * sprites need to reference some internal data structures and have slightly difference code
 * in order to handle clipping to the playfield bounds.
 * 
 * The core sprite drawing approach is the same (set up Bank 1 direct page and stack), but
 * the setup and dispatch are a bit different.
 * 
 * A Note on Clipping
 * 
 * GTE supports two clipping buffers for sprites to use.  The first one is a static buffer
 * that is aligned with the playfield and is used to clip the sprite when crossing the
 * left and right boundaries, but since it's a static image, mask data can be put anywhere
 * that the sprites should not show through, so irregular borders and sprite punch-outs
 * on the playfield are possible.
 * 
 * The second buffer matches the current tiles in the playfield and can be used as a 
 * dynamic mask of the playfield.  Since the sprite code itself must use this data,
 * different variations of the same sprite can be created to stand in "front" and "behind"
 * different screen elements.
 * 
 * The sprite requires the X and Y registers for this.  The most general code that
 * should be used for each sprite word is this:
 * 
 *   Example: DATA = $5670, MASK = $000F, screen_mask = $FF00, field_mask = $F0FF
 *   lda DP                 ; A = $1234
 *   eor #DATA              ; A = $4444
 *   and #~MASK             ; A = $4440
 *   and screen_mask,y      ; A = $4400
 *   and >field_mask,x      ; A = $4000
 *   eor DP                 ; A = $5234  <-- Only the high nibble is set to the sprite data
 *   sta DP
 * 
 * It is not *required* that sprites use this approach, any compiled sprites code can be used,
 * so if a sprite does not need to be masked, than any of the fast sprite approaches can be
 * used.
 *
 * For clipping vertically, we pass in the starting and finishing lines in a register and
 * a sprite record is set up to allow the sprite to be entered in the middle and exited
 * before the last line of the sprite.
 */