pico: Delcare screen_fb as uint16_t

[Daft-Freak]

Everything that isn't blit::Surface wants that and we assume 2-byte alignment in the blending code.

Seems to fix #755, which was hardfaulting in the blending code.

[ali1234]

This fixes it for me. Is declaring the framebuffer uint16 guaranteed to align it or should we take further measures eg alignas?

I think this fix is correct because adding another static bool to the example also makes the crash go away, due to bringing everything back into alignment. It also explains why both C and CXX flags must be Os in order for the bug to happen, because any aligned variables between the static bool and the framebuffer will push everything after back into alignment.

[Daft-Freak]

I think it should be enough, any attempt to access an unaligned uint16 causes a fault... so I'd hope the compiler is smart enough to not do that.

Though forcing it to be aligned to 4 bytes may be (slightly) more optimal...

[Gadgetoid]

Thank you all! This actually reads better, too, albeit the number of uint8 casts seems to have balanced it out 😆