feat(qt6): QOpenGLWidget-based plot widget and demo

adapters/qt6/CMakeLists.txt

@@ -0,0 +1,37 @@
+# The Qt6 adapter: a QOpenGLWidget that renders a plot into its own framebuffer
+# and drives it from Qt's events. It is the third host, and the first whose
+# toolkit already owns a GL widget -- so the interesting parts are the surface
+# format the application must have set before the widget exists (documented on
+# the class) and the device-pixel-ratio mapping between Qt's logical coordinates
+# and the framebuffer pixels the library speaks.
+#
+# Unlike the GLFW and ImGui adapters this one is not fetched: Qt is a system SDK,
+# and a project that enables this target is asking for the installed one.
+find_package(Qt6 QUIET COMPONENTS Core Gui Widgets OpenGLWidgets)
+
+if(NOT Qt6_FOUND)
+  message(FATAL_ERROR
+    "plot: PLOT_BUILD_QT6 is ON but Qt6 was not found (need the Core, Gui, Widgets"
+    " and OpenGLWidgets components). Install Qt6 -- on Arch, qt6-base -- or"
+    " configure with -DPLOT_BUILD_QT6=OFF, or point CMAKE_PREFIX_PATH at the Qt6"
+    " installation.")
+endif()
+
+add_library(plot_qt6 STATIC qt_plot_widget.cpp qt_plot_widget.hpp)
+add_library(plot::qt6 ALIAS plot_qt6)
+# The public include root is `adapters/`, so the header is reached as
+# <qt6/qt_plot_widget.hpp> -- the toolkit's name first, like every other adapter's.
+target_include_directories(plot_qt6 PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/..")
+# Qt is PUBLIC, unlike GLFW and ImGui: the widget's header derives from
+# QOpenGLWidget, so a consumer needs Qt's headers and libraries to use it at all.
+target_link_libraries(plot_qt6 PUBLIC
+  plot::core
+  Qt6::Core
+  Qt6::Gui
+  Qt6::Widgets
+  Qt6::OpenGLWidgets)
+# The widget declares Q_OBJECT, so its header has to go through moc. The property
+# is set on this target rather than globally: no other target in the project has a
+# Qt class in it, and AUTOMOC scans every source of a target it is set on.
+set_target_properties(plot_qt6 PROPERTIES AUTOMOC ON)
+plot_apply_warnings(plot_qt6)