Allows for finer grained timeout values, and fixes a FIXME.
This also drops the legacy select() fallback path in favor of presuming that poll() is always available. poll() is part of the POSIX.1-2001 standard, has been available in Unix since some time in the 1980s, the BSDs since at least the early 90s, and Linux since kernel 2.1, which predates kernel support for Pthreads. glibc also has its own emulation using select(), if necessary.
- Use modern Xkb functions where appropriate and cleanly separate the modern and legacy paths.
- Remove the deprecated XKeycodeToKeysym function in favor of directly querying the keymap on the legacy path.
- Look up virtual modifiers by name on the Xkb path to better handle remapping (equivalent to the modifier handling under Wayland).
- Optimize keymap creation on the Xkb path to cut keymap build times and enable fast group switching (equivalent to keymap handling on Wayland).
- Enable and handle Xkb events to handle changes to the group, mapping, and modifier states. This is more reliable than using the legacy events (group changes may not arrive if the window lacks pointer focus), and better handles cases where modifiers are latched, locked, or activated externally rather than physically pressed.
xkbcommon 1.10.0 declared certain modifier names to be deprecated, and the current plan is to remove them in 1.12.0. Use the new recommended names and modifier mask retrieval function when building against version 1.10.0 and higher.
Currently, `SDL_DEFAULT_ASSERT_LEVEL` is commented out by CMake when its value is 0, setting the assertions level to the default value instead of disabling them.
This change:
- defines `SDL_DEFAULT_ASSERT_LEVEL_CONFIGURED` when its value is non-zero.
- defines `SDL_DEFAULT_ASSERT_LEVEL`, regardless of its value, when `SDL_DEFAULT_ASSERT_LEVEL_CONFIGURED` is defined.
This can be reverted if a toolchain arrives that can handle C99 features like
variables declared in the middle of a scope, but for now we literally can't
compile SDL3 for this platform.
Fixes#11243.
