Implemented high-DPI support

[CobaltXII]

This is done by initializing the SDL_Window with the SDL_WINDOW_ALLOW_HIGHDPI flag. Alone, that flag is not enough. So, I use SDL_GL_GetDrawableSize() to fetch the real size of the window. I then create two 'scaling factor' variables, one for each dimension. The 'scaling factor' variables are calculated by dividing the output of SDL_GL_GetDrawableSize by the original window size... i.e. pixels_width, pixels_height. Then, in the function SDL_ReDraw (by Bisqwit), the destination rectangle (trect) is scaled by the scaling factors, creating a smooth, crisp display on high-DPI monitors.