Fix handling of top two pixel rows in be_blur_c

5dd56af2 replaced our implementation of \be with xy-VSFilter's.
This error is not present in the xy-VSFilter code; it was introduced
by accident in a merge that was later squashed as part of 5dd56af2.

Note: the new \be reads in but does not write out
the first and last row and column of pixels.
Remove dirty pixels from ASM be_blur output

A loop initializer was missing, so output started one row too early.
A loop condition check was missing, so output sometimes stopped
one column too late.

Also remove a couple of dead assignments.
drawing: parse numbers as floats

Parse values as float and convert into 26.6 fixed point. This actually
simplifies scaling a bit.

Fixes #63.
Merge pull request #66 from grigorig/drawing-floats

drawing: parse numbers as floats
shaper: fix nasty sign bug in font functions

Fixes issue #68.
Don't crash on \fscx0 with non-zero \blur

Regression in commit 98707694.
Do font supersampling the VSFilter way

To avoid jerking when font size scale is animated caused by grid
fitting, we intentionally request an overly large font size from
FreeType and downscale the outline afterwards.

Currently this large font size is fixed at 256.
This commit changes it to 64 times the actual wanted font size,
which is what is used by VSFilter. The advantages are:
  * Compatibility with VSFilter.
  * Support for font hinting, even if at 64 times the size.
    Knowing this coefficient, scripts can compensate
    for it by correspondingly decreasing \fs values.

This improves upon commits 7af7801 and c207000.

The drawback is that jerking is reintroduced when an mplayer
window with libass EOSD is resized. Jerking could be avoided
by scaling \fscx and \fscy instead of \fs. Pro: this would make
the exact look of the font completely resolution-independent.
Con: this would play badly with font hinting. In any case,
this commit makes no attempt to do this.

ensure_font_size, unused since 7af7801, is removed in favor of
clipping the font size just before passing it to FreeType, as
discussed in issue #46. The mysterious height*2 check is dropped.
Scale \fscx/\fscy instead of \fs from storage to display resolution

This prevents jerking on media player window resize.