Fix hair/curve drawing artifacts when workarounds are enabled.

Regression introduced by {rB601995c3b86986cf8f8e5b6e5a65bcfa7f8f2e32}.
Noticed by Heist project as they render final frames with workarounds enabled.

The mentioned patch introduces attaching VBO as textures. This used to be
done by the caller. The mechanism used a different order hence the VBO
could still be unbound when using. This cannot be solved inside the new
mechanism clearly so this patch will just bind when the buffer isn't bound
just before the drawing command is sent to the GPU driver.

source/blender/gpu/opengl/gl_shader.cc

@@ -1051,6 +1051,10 @@ bool GLShader::transform_feedback_enable(GPUVertBuf *buf_)
 
   GLVertBuf *buf = static_cast<GLVertBuf *>(unwrap(buf_));
 
+  if (buf->vbo_id_ == 0) {
+    buf->bind();
+  }
+
   BLI_assert(buf->vbo_id_ != 0);
 
   glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, buf->vbo_id_);