`VFPU_CONTEXT` might not be initialized for further gum calls

[nitwhiz]

I tried following a psp graphics programming tutorial and on PPSSPP it ran fine, as long as illegal memory access is ignored. On the PSP, it crashes.
If illegal memory accesses aren't ignored, it crashes on PPSSPP, too.

With psplink I found that psp::sys::sceGumMatrixMode prints access errors over and over again.

After digging through psp::sys::gum, I saw that if psp::sys::sceGumLoadIdentity or psp::sys::sceGumLoadMatrix weren't called before other gum methods using the VFPU_CONTEXT, there would be no VFPU_CONTEXT to access (as it's None).

Only these 2 methods are calling VFPU_CONTEXT.get_or_insert_with instead of psp::sys::gum::get_context_unchecked.