text syntax highlighting, add utility function 'text_format_fill()' which fills in the line with a formatting value.

this fixes a mistake in OSL lexer which would comment all lines after '//'


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53834 954f8c5b-7b00-dc11-b283-0030488c597c