This adds glfwGetWindowContentScale and glfwGetMonitorContentScale for
querying the recommended drawing scale factor for DPI-aware rendering.
Parts of this patch are based on code by @ferreiradaselva.
Fixes#235.
Fixes#439.
Fixes#677.
Fixes#845.
Fixes#898.
This completes support for window framebuffer transparency on Windows,
macOS and X11. Note that the hint/attribute may be renamed before
release to clarify its relationship to GLFW_OPACITY.
Fixes#197.
Closes#1079.
Related to #663.
Related to #715.
Related to #723.
Related to #1078.