ColorPicker: ignore TextureRect expand mode during legacy picking

The color picker has a "legacy" fallback mode when
`FEATURE_SCREEN_CAPTURE` is not supported by the current
`DisplayServer`. It works by "freezing" the current view by making a
huge `Popup` with a `TextureRect` inside covering the screen.

Before this patch, said `TextureRect` could get cut off if its buffer
was nominally bigger than the parent popup, such as when the
`canvas_items` content scale mode is active.

This is because the default TextureRect scaling logic only allows
expanding the texture up. `EXPAND_IGNORE_SIZE` fixes that by allowing
the texture to logically "shrink", filling the whole screen
independently of the actual buffer size.

Special thanks to Federico Fausto Santoro for helping with the diagnosis
:D

Co-Authored-By: Federico Fausto Santoro <[email protected]>

scene/gui/color_picker.cpp

@@ -1556,6 +1556,7 @@ void ColorPicker::_pick_button_pressed_legacy() {
 
 		picker_texture_rect = memnew(TextureRect);
 		picker_texture_rect->set_anchors_preset(Control::PRESET_FULL_RECT);
+		picker_texture_rect->set_expand_mode(TextureRect::EXPAND_IGNORE_SIZE);
 		picker_window->add_child(picker_texture_rect);
 		picker_texture_rect->set_default_cursor_shape(CURSOR_POINTING_HAND);
 		picker_texture_rect->connect(SceneStringName(gui_input), callable_mp(this, &ColorPicker::_picker_texture_input));