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/backend/backend.go

@@ -156,6 +156,7 @@ type Texture interface {
 
 const (
 	DepthFuncGreater DepthFunc = iota
+	DepthFuncGreaterEqual
 )
 
 const (