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This subroutine is called from S_POINT. It is entered with the coordinates of a pixel on the calculator stack, and returns a last value of 1 if that pixel is ink colour, and 0 if it is paper colour.
POINT_SUB 08907 CALL STK_TO_BC y-coordinate to B, x to C.
08910 CALL PIXEL_ADD Pixel address to HL.
08913 LD B,A B will count A+1 loops to get the wanted bit of (HL) to location 0.
08914 INC B
08915 LD A,(HL)
POINT_LP 08916 RLCA The shifts.
08919 AND 1 The bit is 1 for ink, 0 for paper.
08921 JP STACK_A It is put on the calculator stack.
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