fastuidraw/gl_backend/painter_backend_gl: use glClearBufferfv to clear depth buffer
fastuidraw/gl_backend/painter_backend_gl: use glClearBufferfv to clear depth buffer

Using glClearBufferfv() instead of glClear() also allows us to avoid
touching more GL global state: glClearDepth value.

01org/fastuidraw,01org/fastuidraw