Fix functionality tests on desktop GL.
Fix functionality tests on desktop GL.

The attribute `a_1` may be assigned generic vertex attribute 0, which
has some magical properties on legacy desktop GL. By assigning the
attribute locations ourselves, we can avoid the magic index 0.

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