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")
 	}

gpu/gl/gl.go

@@ -35,6 +35,7 @@ const (
 	INFO_LOG_LENGTH                       = 0x8B84
 	INVALID_INDEX                         = ^uint(0)
 	GREATER                               = 0x204
+	GEQUAL                                = 0x206
 	LINEAR                                = 0x2601
 	LINK_STATUS                           = 0x8b82
 	LUMINANCE                             = 0x1909