Fix segfault on certain buffer sizes

[nia-e]

This was a lovely (awful) issue to fix, and was actually the core issue underpinning #74. When implementing the lv_disp_t, the LVGL docs specify that the driver object must be the first member of the struct. In #67 I changed the struct we have here to also hold the buffer, as this simplified both the API and the internal logic. However, I was not aware of the requirement re: struct layout, and Rust doesn't actually guarantee any specific type layout; so, on certain buffer sizes, the compiler was putting the buffer before the driver.

On the bright side, all this segfault chasing has quadrupled my productivity with lldb.

[rafaelcaricio]

I need to step up my game and learn how to use lldb 😅

[nia-e]

It's so painful yet so worth it