clx: free associated xrender picture when destroying pixmaps
clx: free associated xrender picture when destroying pixmaps

Some code (at least MCCLIM-TRUETYPE::DRAWABLE-PICTURE) associates an
XRender picture with a pixmap by stashing it into the pixmap's
plist. Prior to this change, the picture got leaked when the pixmap
was destroyed.

slyrus/mcclim,slyrus/mcclim