feat(gtk4): GtkGLArea-based plot area and demo

tests/CMakeLists.txt

@@ -56,6 +56,11 @@ endif()
 # tested without linking Qt -- and without `PLOT_BUILD_QT6`, since no Qt target has
 # to exist for it to compile.
 target_sources(plot_tests PRIVATE test_qt_pointer.cpp)
+
+# Same for the GTK adapter's input conversion (see adapters/gtk4/gtk_input.hpp):
+# the wheel's unit and sign and the modifier mask are this project's logic, and a
+# headless X server can only deliver the discrete-wheel branch of it.
+target_sources(plot_tests PRIVATE test_gtk_input.cpp)
 target_include_directories(plot_tests PRIVATE "${CMAKE_SOURCE_DIR}/adapters")
 
 # Same switches the library is built with: the cost tests read the counters