fix ofImageType retrieval issue in ofTexture.readToPixels()

fixes an issue where readToPixels() would by default receive an incorrect ofImageType.

````ofGetGlTypeFromInternal(texData.glTypeInternal)```` returns a GL type as in: ````GL_UNSIGNED_BYTE```` etc,
whilst ````ofGetImageTypeFromGLType( parameter )```` expects as parameter something along the lines of ````GL_RGB```` or ````GL_RGBA````, in almost all cases returned ````OF_IMAGE_UNDEFINED````, even when attempting to allocate a basic ````RGBA```` image.

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>