drawing: doc: Add block characters to test file
drawing: Fix drawing of U+2594 to align with U+2587

The upper ⅛ block was misaligned with the lower ⅞ block.

This also fixes part of https://bugzilla.gnome.org/show_bug.cgi?id=709584
drawing: Fix drawing of the n-eights block characters

Make the n-eights block characters align with each other and the half
block characters as well.

https://bugzilla.gnome.org/show_bug.cgi?id=709584