FreeTypeFont: use proper image format for glyph cache in ES.

Use ImageFormat::Red if EXT_texture_rg is supported, fallback to
ImageFormat::Luminance otherwise. Issues with Luminance and
DistanceField do not apply here.
TgaImporter: more verbose error when image type is not supported.
TgaImporter: don't include these headers if not really needed.
TgaImageConverter: ported to OpenGL ES.
TgaImageConverter: don't forget about endianness.
TgaImporter: removed unneeded #include.