Manage AltGr key combinations in windows backends

[rcases]

On my keyboard, the '#' character is generated by the combination 'AltGr'+'3' in the same way that on many European keyboards the '€' symbol is generated by 'AltGr'+'e'
For the reason explained in a previous comment in the code, this combination is prohibited in sokol_imgui.h, line 3132.

Commenting out that line makes the problem disappear, although I think a better solution would be to put some mask on the key modifiers in the Windows backend to prevent the "AltGr" modifier from being sent in the SAPP_EVENTTYPE_CHAR event.

[floooh]

Hmm, I don't have an immediate idea for a robust solution, but there's a related recent ticket here:

#1079

...the code almost certainly exists because of combinations like Ctrl+X/C/P, so maybe we can just remove the ALT modifier from the mask.

[rcases]

Modifiers could be removed when ALT and Ctrl are used together.
Look at your WebGL imgui example in Inputs & Focus.
Key events that generate characters (WM_CHAR ) do not transmit modifiers.
Ctrl+X/C/P combinations are supposed to be managed through KeyDown