texture-pixmap-x11: Move GLX code to cogl-winsys-glx.c

texture-pixmap-x11: Move GLX code to cogl-winsys-glx.c

This moves the GLX specific code from cogl-texture-pixmap-x11.c into
cogl-winsys-glx.c. If we want the winsys components to by dynamically
loadable then we can't have GLX code scattered outside of
cogl-winsys-glx.c. This also sets us up for supporting the
EGL_texture_from_pixmap extension which is almost identical to the
GLX_texture_from_pixmap extension.