Move `Graph_Alloc` and open/close Disps to `gfx.h`

[Dragorn421]

which is where the GraphicsContext struct is defined.

(Note: imo not all Graph_ functions belong in gfx.h)

[mzxrules]

wanna note this conflicts with #1972

I'm more of the opinion that this stuff belongs in Graph.h not Gfx.h because the GraphicsContext belongs to the graph thread.

[Thar0]

graph.h and gfx.h should be the same header 🚀

[Dragorn421]

Based on this having received approvals I'll prefer and merge this over #1972 Will merge in 24 hours

[cadmic]

I don't think this necessarily replaces #1972, that has a few more changes

[Dragorn421]

Right, for the conflicting part I mean