Improve safety of `get_quad_context`

[e2-71828]

This uses a thread local variable to ensure that the miniquad::Context doesn't cross a thread boundary. It also enforces the mutable reference aliasing rules by nulling the pointer when it is borrowed and restoring it when the borrow ends

This uncovered one instance of UB in cargo test due to an aliased mutable reference which has been fixed. There may be others which aren't covered by the test suite; they will now panic.

This is a breaking change because the type of get_internal_gl has changed to include a guard object instead of an unbounded mutable reference; downstream users will need to adjust their code a bit (see the raw_miniquad and particles changes for an example of what this change may look like).