Texture: add targa support (.tga)

[meszaros-lajos-gyorgy]

Arx supports it: https://github.com/arx/ArxLibertatis/blob/ArxFatalis-1.21/Sources/EERIE/EERIETexture.cpp#L749

[meszaros-lajos-gyorgy]

Sharp doesn't support it out of the box and there's no plugin for it readily available, like sharp-bmp