Fix: Cast loop variable to ImU32 in ImPlot3DColormapData to fix C4365 warning

[JunkyoLee]

The build error

In implot3d_internal.h line 266, the for-loop counter s is declared as int but is passed directly to ImMixU32(ImU32, ImU32, ImU32), causing a C4365 signed/unsigned mismatch warning on MSVC. When warnings are treated as errors (e.g. /W4 /WX builds), this breaks compilation.

implot3d/implot3d_internal.h(266,56): error C2220: the following warning is treated as an error
                    ImU32 c = ImPlot3D::ImMixU32(a, b, s);
                                                       ^
implot3d/implot3d_internal.h(266,56): warning C4365: 'argument': conversion from 'int' to 'ImU32', signed/unsigned mismatch

Fix (applied): Cast s to ImU32

Instead of suppressing this warning, I addressed the root cause in the code to ensure a more robust solution.

// Before
ImU32 c = ImPlot3D::ImMixU32(a, b, s);
// After
ImU32 c = ImPlot3D::ImMixU32(a, b, (ImU32)s);

Alternative considered: Changing the loop variable itself to ImU32:

for (ImU32 s = 0; s < 255; ++s) {

[brenocq]

LGTM, thanks for the fix @JunkyoLee!