This removes all dependencies from the GLFW test programs on the Vulkan
SDK.
It also removes support for linking the GLFW shared library (dynamic
library, DLL) against the Vulkan loader static library.
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.