gtk/gl: Only unbind buffers/vertex attrib arrays if we can't directly bind the vertex array to 0

Binding the vertex array to 0 will unbind everything else already.

In the previous order older versions of the Intel GL driver caused
errors to be printed for every single call when disabling the vertex
attrib arrays after binding the vertex array to 0.
8 jobs for unbind-before-voa in 18 minutes and 12 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #82400
gst indent

00:00:32

passed #82399
manifest

00:00:39

 
  Build
passed #82401
build fedora x86_64

00:04:49

 
  Test
passed #82402
check fedora

00:04:38

 
  Full Builds
passed #82403
build android arm64 api28

00:02:41

passed #82405
build cerbero cross win32

00:08:04

passed #82406
build cerbero cross win64

00:05:40

passed #82404
build cerbero fedora x86_64

00:05:56