The LTDC include file was defined with combined bit
semantics and bit position. As a result instead of
LTDC_GCR_VSPOL which is the bit which defines vertical
sync polarity, this had been defined to be
LTDC_GCR_VSPOL_LOW (0) and LTDC_GCR_VSPOL_HIGH (non zero).
This sort of define makes it impossible to know ahead of
time what operation would set or reset the bit (some are
negative logic, others are postive logic, so affirmative
defines could mean either "set the bit" or "reset the bit"
I've added the non-semantic bit define so that it is clear
in my code if the bit is being set or reset.
Discussion took place at https://github.com/libopencm3/libopencm3/pull/889
1e9a2e641c