gpu: saturate instead of overflowing depth buffer

Use greater-than-or-equal test and saturate the z depth buffer
when more than 65k objects are drawn.

Fixes gio#127

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/gl/backend.go

@@ -399,6 +399,8 @@ func (b *Backend) DepthFunc(f backend.DepthFunc) {
 	switch f {
 	case backend.DepthFuncGreater:
 		glfunc = GREATER
+	case backend.DepthFuncGreaterEqual:
+		glfunc = GEQUAL
 	default:
 		panic("unsupported depth func")
 	}