Add example for multi-texturing
Add example for multi-texturing

I added this while trying to find the bug in the sdf test. Funny thing
is that the error was that texture handles were used where I should have
used texture units. When there is only one program involved, these
are equal. Therefore this example did not reveal the bug. Still nice
to have though ...

jdreaver/vispy,srinathv/vispy,sbtlaarzc/vispy,QuLogic/vispy,Eric89GXL/vispy,dchilds7/Deysha-Star-Formation,jdreaver/vispy,sh4wn/vispy,RebeccaWPerry/vispy,srinathv/vispy,bollu/vispy,Eric89GXL/vispy,inclement/vispy,michaelaye/vispy,hronoses/vispy,ghisvail/vispy,dchilds7/Deysha-Star-Formation,hronoses/vispy,bollu/vispy,kkuunnddaannkk/vispy,michaelaye/vispy,sbtlaarzc/vispy,QuLogic/vispy,sh4wn/vispy,ghisvail/vispy,jay3sh/vispy,inclement/vispy,kkuunnddaannkk/vispy,julienr/vispy,drufat/vispy,RebeccaWPerry/vispy,jay3sh/vispy,RebeccaWPerry/vispy,michaelaye/vispy,ghisvail/vispy,hronoses/vispy,dchilds7/Deysha-Star-Formation,bollu/vispy,QuLogic/vispy,sh4wn/vispy,kkuunnddaannkk/vispy,Eric89GXL/vispy,drufat/vispy,jay3sh/vispy,inclement/vispy,sbtlaarzc/vispy,julienr/vispy,drufat/vispy,julienr/vispy,jdreaver/vispy,srinathv/vispy