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 Name Job ID Coverage
  Preparation
passed gst indent #82400

00:00:32

passed manifest #82399

00:00:39

 
  Build
passed build fedora x86_64 #82401

00:04:49

 
  Test
passed check fedora #82402

00:04:38

 
  Full Builds
passed build android arm64 api28 #82403

00:02:41

passed build cerbero cross win32 #82405

00:08:04

passed build cerbero cross win64 #82406

00:05:40

passed build cerbero fedora x86_64 #82404

00:05:56