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>

app/internal/d3d11/backend_windows.go

@@ -605,6 +605,8 @@ func (b *Backend) prepareDraw(mode backend.DrawMode) {
 		switch b.depthState.fn {
 		case backend.DepthFuncGreater:
 			desc.DepthFunc = _D3D11_COMPARISON_GREATER
+		case backend.DepthFuncGreaterEqual:
+			desc.DepthFunc = _D3D11_COMPARISON_GREATER_EQUAL
 		default:
 			panic("unsupported depth func")
 		}

app/internal/d3d11/d3d11_windows.go

@@ -621,7 +621,8 @@ const (
 
 	_D3D11_DEPTH_WRITE_MASK_ALL = 1
 
-	_D3D11_COMPARISON_GREATER = 5
+	_D3D11_COMPARISON_GREATER       = 5
+	_D3D11_COMPARISON_GREATER_EQUAL = 7
 
 	_D3D11_BLEND_OP_ADD        = 1
 	_D3D11_BLEND_ONE           = 2