Parse angle in styles as double number

Similar to VSFilter. Thanks to Iroha for reporting this issue.

Note: this breaks the ABI!
Fix fallback event formats

Fixes issue 83.
Add Blur field to ASS_Style

Useful when applications specify the style manually for non-ASS
subtitles.
Scale border size with user font size coefficient
Make closing ')' optional for some ASS tags

It appears VSFilter generally accepts the tag anyway if closing ')' are
missing in tags like \pos(1,2). Since they're not strictly needed
anyway, make the last ')' optional in order to make parsing more
tolerant against broken subs.

Fixes issue #79.
style overrides: subtitle styling override
style overrides: do not use user font scale coefficient for positioned events
style overrides: selective style override doesn't apply to events with margins
style overrides: don't apply line position user setting to events with no style overriding

A previous commit added the possibility that a normal (unpositioned) event
is considered as not suitable for selective style overriding. This makes it
necessary to add a special case to the user line position code to leave
these events unaffected.