Fix shift-sign issue in Convert.c

[r-barnes]

Fixes

libImaging/Convert.c:513:25: error: signed shift result (0xFF000000) sets the sign bit of the shift expression's type ('int') and becomes negative [-Werror,-Wshift-sign-overflow]
    UINT32 trns = (0xff << 24) | ((b & 0xff) << 16) | ((g & 0xff) << 8) | (r & 0xff);
                   ~~~~ ^  ~~

[radarhere]

Hi. A quick glance through some of our CI jobs, and I don't see this problem on main. Could you provide some more information about the environment where you're seeing this?

[r-barnes]

@radarhere - We compile pillow using a separate build system to get better LTO.

This build system has -Wshift-sign-overflow enabled for code safety reasons (it catches bugs!). It's therefore advantageous to have as much upstream code as possible be able to compile safely.

You could enable -Wshift-sign-overflow in pillow, though I don't know how to do this myself, but, in the meantime, godbolt demonstrates the failure indicated above.

[nulano]

You could enable -Wshift-sign-overflow in pillow, though I don't know how to do this myself,

Generally, compiler flags specific to CI are listed here

Pillow/Makefile

Line 63 in e42528e

CFLAGS="-coverage -Werror=implicit-function-declaration" python3 -m pip -v install .

but it seems this warning is not supported by GCC, only Clang.