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>
Merge pull request #2759 from tgfrerer/fix_ofTexture_readToPixels

fix ofImageType retrieval issue in ofTexture.readToPixels()