Win32: Ignore WM_MOUSELEAVE in relative mode.

We get an WM_MOUSELEAVE when we switch to relative mode, even though the cursor is still in the window.
Ignoring this event to not end up with a NULL mouse focus.

This fixes http://bugzilla.libsdl.org/show_bug.cgi?id=1861
Win32: Fix issue with SetCapture & negative values.

This fixes an issue where we were using the wrong macros to extract the position from WM_MOUSEMOVE, so negative values were behaving incorrectly.
These would be generated in multimon situations, or if you use SetCapture.

Fixes http://bugzilla.libsdl.org/show_bug.cgi?id=1175