Define static functions fill_image_linesize() and

fill_image_data_ptr(). ff_fill_linesize() and ff_fill_pointer() now wrap
these functions.

The new functions are more generic, and are going to be exported in a
future patch.

Patch by S.N. Hemanth Meenakshisundaram smeenaks # ucsd § edu.

Originally committed as revision 24512 to svn://svn.ffmpeg.org/ffmpeg/trunk