Modify blit segment spec

[ddelemeny]

The indexing scheme implemented to address spritesheet pages with the blit_segment feature feels a bit tedious, as keeping in mind that odd and even values mean BG and FG respectively and alternating indices requires some mental gymnastics.

This PR implements another way :
Page indexing goes from left to right then top to bottom, with index offsets to target bpp
This should feel more natural as it is the same indexing scheme as tiles.

Here's the revised full spec:

0000 SYS GFX
0001 FONT

0010 4bpp BG Page 0
0011 4bpp FG Page 0

0100 2bpp BG Page 0
0101 2bpp BG Page 1
0110 2bpp FG Page 0
0111 2bpp FG Page 1

1000 1bpp BG Page 0
1001 1bpp BG Page 1
1010 1bpp BG Page 2 
1011 1bpp BG Page 3
1100 1bpp FG Page 0
1101 1bpp FG Page 1
1110 1bpp FG Page 2
1111 1bpp FG Page 3

Editor and demo are modified accordingly.

[nesbox]

Thanks