Improve Reph's default spacing

[lianghai]

Because text engines generally don't limit context for Sinhala Reph, the glyph Reph is produced as soon as character sequence <RA, Virama, ZWJ> appears. Also, dottedcircle doesn't seem to be inserted by text engines.
If the default spacing of Reph is optimized for anchoring-less positioning on base glyphs, it's on the left of curser, which is confusing since it looks like it belongs to the last syllable (or, might even not be visible if the curser is on the beginning of a line — although Reph can't really start a word).

Therefore, it might be a better idea to center the outline of Reph to its zero-advance so it appears upon the curser.
However, when GPOS mark-positioning / anchoring is active, the glyph Reph gets positioned on the preceding consonant, regardless of the default spacing of Reph itself.

[davelab6]

@lianghai what is the status of this? :)

[davelab6]

@lianghai ping