bg data

BG data bitset
----\
b00  |
---  |
b01  |
---  |
b02  |
---  |
b03  |
---  |
b04  |
---  +-BG tile number
b05  |
---  |
b06  |
---  |
b07  |
---  |
b08  |
---  |
b09  |
-----+
b10  |
---  +-Spare bits
b11  |
----<
b12  |          /-Rotate by 90
---  +-Rotation<
b13  |          \-Rotate by 180
----<
b14  |      /-Flip horizontal
---  +-Flip<
b15  |      \-Flip vertical
----/