Enable FIX_ARTIFACTS_BY_STRECHING_TEXEL by default

[1scaR1]

It seems it was disabled accidentally in one of the latest commits.

[pandamicro]

FIX_ARTIFACTS_BY_STRECHING_TEXEL may cause LabelBMFont texture being cut off some pixels at the top and bottom, it's normally not recognizable in other textures, but in LabelBMFont it's more annoying. It's useful to remove black pixel gap when people use tiled map, so this value should be set by user when needed.

Previously it's enabled by default because it can also fix texture artifacts when hard edge is required for sprite frame in atlas. But that should be fixed by texture.setAliasTexParameters(), because it's due to the linear texture filter, AliasTexParameters is using nearest texture filter.