wgpu UB fixes

[r-lyeh]

As discussed in #9246, this PR addresses UB bugs in the WGPU examples (note that WGPUCallbackMode is never 0). Also fixes setup regarding future/asyncs.

typedef enum WGPUCallbackMode {
    WGPUCallbackMode_WaitAnyOnly = 0x00000001,
    WGPUCallbackMode_AllowProcessEvents = 0x00000002,
    WGPUCallbackMode_AllowSpontaneous = 0x00000003,
    WGPUCallbackMode_Force32 = 0x7FFFFFFF
} WGPUCallbackMode WGPU_ENUM_ATTRIBUTE;

[ocornut]

Merged as b5ccee7, thank you!

(For next time, note that it would also work if you were split this into an individual commit as part of the exist PR. I can check selectively cherry-pick a commit even if don't merge all the PR in one go.)