fix(gum): create the VFPU context in whichever gum function runs first

[ItsNoHax]

Fixes #189.

The bug

VFPU_CONTEXT is created by sceGumLoadIdentity and sceGumLoadMatrix only. Every other gum
function reaches it through get_context_unchecked, which resolves a None context with
core::intrinsics::unreachable. Opening with any of them is undefined behaviour, and on a
debug build it traps:

E CPU: CPU exception: break instruction hit at 0881ad9c [ci_tests.text+16d9c]
psp::sys::gum::get_context_unchecked [ci_tests.text+16d2c]
sceGumMatrixMode [ci_tests.text+16e6c]

That is the report's sceGumMatrixMode failure exactly. It is easy to hit because
sceGumMatrixMode is what most setup code calls first — as in the issue's tutorial, which selects
a matrix mode before loading anything into it.

The fix

get_context_unchecked becomes get_context, which creates the context on demand, and the two
functions that were doing that inline now go through it too. Creating a Context is a zeroed
matrix set with an empty saved mask, so doing it from any entry point is the same work
sceGumLoadIdentity already did — the choice of which function got to do it was arbitrary.

Tests

One case in ci/tests: open with sceGumMatrixMode on a cold context, then build a translation
and read it back with sceGumStoreMatrix to show the context it created is usable. No rendering,
so it needs no GU.

Only the first gum call in a process is cold, so there is one case to be had and it has to run
before the rest of the suite — hence its slot at the head of the list in main.rs.

Before (master, a8be764) the run stops at the cold call and never reaches a final token, so
run-tests.sh fails on the missing FINAL_SUCCESS:

STARTING_TESTS

After:

STARTING_TESTS
[PASS]: (gum_cold_matrix_mode) (1.0, 2.0, 3.0) == (1.0, 2.0, 3.0)
...
FINAL_SUCCESS

Run with PPSSPPHeadless ... --timeout=40 -r . as ci/concourse/run-tests.sh does. The rest of
the suite is unaffected and cargo fmt is clean for both psp and ci/tests.

Note

This is independent of #206, which is a separate bug in sceGumPushMatrix. Both add a test module,
so if one lands first the other needs a trivial rebase on the mod and array lines in
ci/tests/src/main.rs. The test here deliberately avoids push/pop so it passes on master's stack
behaviour.