Fix jpeg redraw
Move png.c/h to /image and rework to use the bitmap code
Make RISC OS bitmap struct publically accessible (via riscos/bitmap.h)
Draw export now embeds JPEGs and PNGs/MNGs/JNGs correctly again.
Background images are now plotted correctly again.
svn path=/import/netsurf/; revision=1268
Add generic bitmap functions. Move jpeg and mng code from riscos/ to image/ and update to use bitmap. Note: background tiling and drawfile export for jpeg and mng/png are broken.
svn path=/import/netsurf/; revision=1265