Clarified thread safety statement.
Confined remaining functions to main thread.
Removed superfluous cast.
Fixed MinGW detection.
Fixed crash on monitor disconnection.
Clarified language for file drop callback.
Aligned libGL soname to Linux ABI.

Fixed #261.
Formatting.
Added glfwGetWindowFrameSize.
Added use of frame size to multi-window test.
Formatting.
Added support for _NET_REQUEST_FRAME_EXTENTS.
Formatting.
Split out TLS code into separate modules.

This allows the TLS code to be re-used by partial ports like EGL.