Fix hsync_start computation

nyanpasu64 requested to merge nyanpasu64/libxcvt:fix-hsync-start into master

Previously if mode_info->hsync_start was a multiple of 8 by default (for example on 640x480@60), the code would erroneously add 8 to the beginning of hsync, shortening the hsync duration by 8 pixels. This fixes the bug.

Fixes #6 (closed).

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