[Freetype] TrueType bitmap loader does't ignore xsize like .bdf/.pcf

[Keith Packard]

I'm converting X fonts to .ttf format with Juliusz Chroboczek's fonttosfnt 
tool and have discovered an interesting "feature" of the sfnt code.  The
bdf and pcf loaders both ignore x_ppem, but the sfnt loader requires 
x_ppem to be either 0 or match the x_ppem value in the file.

Well, I'd use '0', but there's no way to get that passed through to the 
loader from the public FreeType API -- Set_Char_Size and Set_Pixel_Sizes 
both compute x_ppem in a way that it can never be zero.

Set_Pixel_Sizes says that the pixel_width/pixel_height are supposed to 
represent an EM square while FT_Bitmap_Size 'width' values are the 
'average width' of the font.  the sfnt loader appears to use the latter in 
matching input to Set_Pixel_Sizes, which belies the documentation for the 
function.  It might be nice to have the sfnt loader really use the 
EM-square rule, but I'm not sure how it could do that.

I guess I'd like to see '0' passed to Set_Pixel_Sizes/Set_Char_Size do the 
'right' thing and somehow manage to pass 0 for x_ppem down to the bitmap 
loader, then that loader could do whatever it wanted.

-keith