Evas fonts: Fix minor deviation in RLE font render

Evas fonts: Fix minor deviation in RLE font render

So I've discovered some weird output values after drawing
some text. The destination alpha would become 0xFE even
when the back buffer had a background with 0xFF alpha.

Example:
Dest is 0xff00ff00 (green).
Color is 0xffffffff (white).
Current font alpha is 170 (0xaa).
--> Output was 0xFEaaFEaa instead of 0xFFaaFFaa.

This is because of some slightly invalid calculation
when doing the font masking (mtab[v] = 0x55 above).

Indeed, MUL_256 takes alpha values in the range [1-256]
and not [0-256] as was assumed.