try to fix problem with fb's odd line_length

For example, on a framebuffer with a picture size of 1366x768, the line
length(bytes per line in fb) may not be 1366*color_depth/CHAR_BIT, but
something larger than this. I guess this is for alignment on the
hardware level.

So the use of screen width in graph_lower functions for determining the
line length should be substituted by global_fix_screen_info.line_length.

But so far the problem is not yet solved.
Revert "try to fix problem with fb's odd line_length"

This reverts commit 9ed112e7e4cf17f648e17e70399ead273afdccfd.

The fix can't be fully tested so far, and is now under develop in branch
fix1366. It will be merged into master as soon as it's ready.