SuperHires mode has a finer granularity of sprite positioning than either
Hires or Lores modes.  This allows for positioning the sprite every other
SuperHires pixel on 70ns boundaries.  The ECS registers SPRxPOS and
SPRxCTL work together as position, size and sprite feature control
registers.  They are usually loaded by the sprite DMA channel, during
horizontal blank, however they may be loaded by the processor.

The two registers are defined as follows:


   SPRxPOS W   A D     Sprite x vertical-horiz start position data

        Bit      Use
        ----     ----
        15-08
        07-00  SH8-SH1  Start horizontal value. Low bit (SH0) in SPRxCTL.


   SPRxCTL W   A D     Sprite x position and control data

        Bit      Use
        ----     ----
        15-08
        07
        06
        05
        04      SHSH1     Start horizontal (SHR mode) 70ns increment
        03      SHSH0     Start horizontal (SHR mode) 35ns (unimplemented)
        02
        01
        00      SH0       Start horiz. value Low bit 140 ns increment

   Note: bits 3 and  4 are in the ECS chips only.

   Warning:
   --------
   70ns sprite positions are only available in SuperHires mode.
   Attempting to use 70ns sprite positioning with Hires mode under
   the current system may lead to unpredictable results.

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