`mouseMoved` bugfix

[MarcinKonowalczyk]

Main changes in updateFramebufferSize to calculate displayPixelRatio and re-use it when not compiling with Emscripten to report a correct mouse position (when compiling for the web the updateFramebufferSize isn't called because the canvas does not get resized). There is a bit of a hack ( in the #if defined(__APPLE__) && !defined(__EMSCRIPTEN__) block) which attempts to detect when the window is dragged between a normal and a retina display (or back), since that was a problem for me.

I believe this somewhat fixes #9.

I've also pulled out the shader code into separate files to make it easier edit and actually use code highlighting on. Absolutely no changes in the shader code.

[MarcinKonowalczyk]

as to the main point of the PR, i'll experiment with glfwSetWindowContentScaleCallback. just, work in progress so far.