... to actual minimum so that the fix for fdo#33090 in
UniscribeLayout::GetNextGlyphs subtracts exactly the right number.

The original value of mnSubStringMin is guessed and may be up to 8
smaller than the actual start of the first VisualItem.

For the bugdoc it happens that sometimes it's too small by 8 and then
the wrong entries in mpGlyphs2Chars are initialized and then apparently
there are single glyphs that represent multiple characters and so
GetNextGlyphs returns a -1 character which is invalid.

 ->GetNextGlyphs dir 1 36 mnSubStringMin 28
 ->GetNextGlyphs g2c #1 [8] = 36
 ->GetNextGlyphs g2c #1 [9] = 37
 ->GetNextGlyphs g2c #1 [10] = 38
 ->GetNextGlyphs g2c #1 [11] = 39
 ->GetNextGlyphs g2c #1 [12] = 40
 ->GetNextGlyphs g2c #2 [4] = 40
 ->GetNextGlyphs g2c #2 [3] = 39
 ->GetNextGlyphs g2c #2 [1] = 38
 ->GetNextGlyphs g2c #2 [1] = 37
 ->GetNextGlyphs g2c #2 [0] = 36
...
 ->GetNextGlyphs init nCharPos -1
 ->GetNextGlyphs g2c [2]  nCharPos -1
 ->GetNextGlyphs set pCharPosAry -1
layout[0]->GetNextGlyphs 768,1024 a1800 c-1 0

Change-Id: Ie33ec797a412aa898bec3f4e8f97b88dcfed4d11