Add a GPU

[fnuecke]

Implement some sort of GPU, probably with VRAM mapped to general address space for access speeds.

All further details TBD.

• Render to screen (requires Add a screen block #7).

[iamgreaser]

Will we use direct VRAM writes, or will we send commands to the GPU instead? I have ideas for both.

[fnuecke]

I can see advantages for both, the first being nice for low-level archs, the latter for high-level ones. The latter would presumably more compact networking (that's why that's what OC does anyway). Not sure if a "hybrid" would be a good idea? Would allow saving some cycles, but that in turn could increase bandwidth use for syncing quite a bit... I have no strong preference right now, tbh, tho I'm leaning towards VRAM writes.

@asiekierka offered to work on this a while ago, tho I'm not sure what his schedule looks like currently, given fabric is a thing now?

[iamgreaser]

Considering that the Sega Master System accesses VRAM via a couple of I/O ports, and that's a Z80-based system, I don't see how a command-based system would be out of place.

Let's get some examples up.

Put string onto screen at given position

LD a, GPU_MSG_PUTSTR
LD de, (ypos<<8)|xpos 
LD hl, str_hello
LD b, str_hello_end - str_hello
OUT (c), a
OUT (c), e
OUT (c), d
OUT (c), b
OTIR

Draw a rectangle

LD a, GPU_MSG_RECTFILL_C4
LD hl, (y<<8)|x
LD de, (h<<8)|w
LD b, color
OUT (c), a
OUT (c), l
OUT (c), h
OUT (c), e
OUT (c), d
OUT (c), b

Actually, just draw a damn thing

LD hl, a_damn_thing
LD b, a_damn_thing_end - a_damn_thing
OTIR