add api useful for handling HiDPI
use doubles, add target, rename canvas -> element
use getBoundingClientRect
move JSEvents dep
remove space
simple test for get_device_pixel_ratio
add docs
add error checking
move get/set_element_css_size to html5.h
Merge pull request #2747 from Daft-Freak/sdl2_apis

add api useful for handling HiDPI