feat(glfw): adapter and 1 MS/s streaming demo

examples/CMakeLists.txt

@@ -0,0 +1,10 @@
+# The example programs. Each one is guarded by the adapter it needs, so
+# -DPLOT_BUILD_EXAMPLES=ON without -DPLOT_BUILD_GLFW=ON is a no-op rather than a
+# configure error.
+find_package(Threads REQUIRED)  # the GLFW demo's producer thread
+
+if(TARGET plot::glfw)
+  add_executable(plot_glfw_demo glfw_demo.cpp)
+  target_link_libraries(plot_glfw_demo PRIVATE plot::glfw Threads::Threads)
+  plot_apply_warnings(plot_glfw_demo)
+endif()