ui/sdl2: release all modifiers

ui/sdl2: release all modifiers

Each virtual console in the SDL2 frontend has a key state map.
When switching windows with GUI keys we have to release all
pressed modifier keys in the currently active window, because
after the switch the now inactive window no longer receives the
key release events.

To reproduce the issue open a text editor in the SDL UI and then
press Ctrl-Alt-2 to open a Compat Monitor Console. Close the
console with the mouse. Try to enter text in the text editor and
notice that the modifier keys Ctrl and Alt are stuck and need to
be pressed once to be released.

Tested-by: Howard Spoelstra <hsp.cat7@gmail.com>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Tested-by: Bernhard Beschow <shentey@gmail.com>
Message-ID: <20240909061552.6122-2-vr_qemu@t-online.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>