Add core_highdpi example

This example enables HIGHDPI and renders a scale showing how the logical
size compares to the pixel size of the window.

examples/Makefile

@@ -508,6 +508,7 @@ CORE = \
     core/core_custom_frame_control \
     core/core_custom_logging \
     core/core_drop_files \
+    core/core_high_dpi \
     core/core_input_gamepad \
     core/core_input_gestures \
     core/core_input_gestures_web \