Creates and initializes the Doors CS GUI system. Sets up all appvars and variables necessary. Repeatedly calling this function will have no negative affects, but the c register will be modified to indicate whether the GUI stack had already been opened previously. The counterpart to this function is the CloseGUIStack routine. Note that the GUI uses several SafeRAM areas that are normally reserved for ASM programs, so your code should be written accordingly.
a = $c7 to use special VAT-swapping speed optimization. If not $c7, the optimization will be omitted. In general, it is safe to avoid explicitly setting the accumulator. It is not recommended that programs use the $C7 mode, as it will offset any pointers into the VAT by 11 bytes.
c = 0 if the stack was closed, 1 if the stack was open.
hl = first data byte of the stack appvar
de = first byte of the VAT data
a, bc, de, hl, Op1