server: Normalize pixel format depth from client

[alistairreay]

Summary

Some VNC clients (e.g. macOS Screen Sharing) report depth=32 in their
SetPixelFormat message for standard RGB888 pixel formats. This causes
calc_bytes_per_cpixel to return 4 instead of 3, corrupting every ZRLE
tile -- the encoder writes 4 bytes per pixel but the client expects 3.

This patch normalizes the depth field using rfb_pixfmt_depth() when
receiving the client's pixel format in on_client_set_pixel_format().
This derives the true depth from the RGB max values (via popcount),
ensuring correct CPIXEL encoding downstream without modifying the
encoding logic itself.

Testing

Tested with macOS Screen Sharing (which sends depth=32) connecting to
wayvnc with ZRLE encoding. Before the fix, the display showed corrupted
flashing tiles. After the fix, the display renders correctly.

Also verified that existing clients sending depth=24 are unaffected --
rfb_pixfmt_depth() returns 24 for standard RGB888, same as what those
clients already report.

I have read and understood CONTRIBUTING.md

[any1]

Did you see this? #160 (comment)

[alistairreay]

Did you see this? #160 (comment)

Ahh yes, thanks for pointing that out. I've reworked the fix to normalize depth using rfb_pixfmt_depth() in on_client_set_pixel_format() instead of modifying the encoding logic. Should be a much cleaner approach.

[any1]

Thanks!