fix(linux): Fix lost context after pressing K_SHIFT

This fixes typing `n>` with sil_ipa keyboard which no longer output
the expected `ŋ` because pressing the shift key reset the context.

For modifier key presses we now exit `KMX_ProcessEvent::ProcessEvent`
early.

This change also removes a redundant line from engine.c. Clearing
the context happens in `reset_context()` again if necessary.

Fixes #5591.