Use C externs for obs-module.h

This allows inter-sharing of functions such as obs_module_text among
both C and C++ files
Rename linux-xshm module to linux-capture
Combine linux-xcomposite and linux-capture (xshm)
linux-capture: Rename pos_* vars to render_*

the pos_x and pos_y variables were somewhat deceptive, because they were
not actually the poition of the cursor.  They represented the position
of the cursor's bitmap on the screen, not the position of the cursor.
linux-capture: Add x/y cursor position vars
linux-capture: Add cursor capture to xcomposite