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

adapters/gtk4/CMakeLists.txt

@@ -0,0 +1,34 @@
+# The GTK4 adapter: a GtkGLArea whose own framebuffer the plot is rendered into
+# and driven from GTK's gestures and event controllers. GTK is a C library whose
+# GL widget cannot be subclassed from C++ (`G_DEFINE_TYPE` declares the type's
+# instance and class structs), so the adapter is a factory that builds an instance
+# and wraps it -- the shape the header documents.
+#
+# Unlike GLFW and ImGui -- and like Qt -- this is not a fetched dependency: GTK4
+# is a system SDK, and a project that enables this target is asking for the
+# installed one. Discovery is pkg-config, and a missing GTK4 fails configuration
+# with the fix in the message.
+find_package(PkgConfig QUIET)
+if(PkgConfig_FOUND)
+  pkg_check_modules(GTK4_PC QUIET IMPORTED_TARGET gtk4)
+endif()
+
+if(NOT TARGET PkgConfig::GTK4_PC)
+  message(FATAL_ERROR
+    "plot: PLOT_BUILD_GTK4 is ON but GTK4 was not found (pkg-config gtk4). Install the "
+    "GTK4 development package -- on Arch, gtk4 -- or configure with -DPLOT_BUILD_GTK4=OFF, "
+    "or point PKG_CONFIG_PATH at the GTK4 .pc file.")
+endif()
+
+add_library(plot_gtk4 STATIC gtk_plot_area.cpp gtk_plot_area.hpp gtk_input.hpp)
+add_library(plot::gtk4 ALIAS plot_gtk4)
+# The public include root is `adapters/`, so the headers are reached as
+# <gtk4/gtk_plot_area.hpp> and <gtk4/gtk_input.hpp> -- the toolkit's name first,
+# like every other adapter's.
+target_include_directories(plot_gtk4 PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/..")
+# GTK is PUBLIC, unlike GLFW and ImGui: the adapter's factory hands out a
+# GtkWidget, so a consumer that embeds the area needs GTK's headers and libraries
+# to put it anywhere -- the widget *is* the toolkit's object -- and GTK's include
+# directory is not one the compiler searches by default.
+target_link_libraries(plot_gtk4 PUBLIC plot::core PkgConfig::GTK4_PC)
+plot_apply_warnings(plot_gtk4)