Add zoom controls to Sprite2DPlugin converter preview

[aXu-AP]

Implements partially godotengine/godot-proposals#8294
Sprite2D's convert tool's preview is hard to make out details if you want to use it for pixel art on a high resolution display:

Or high resolution assets on a standard resolution display:

Also in either case studying closer details is hard without zoom function.

This pr implements zooming controls for the preview. Also the initial view is zoomed in so that the whole texture is in view:

[KoBeWi]

You need to setup ViewPanner:

godot/editor/plugins/polygon_2d_editor_plugin.cpp

Lines 77 to 79 in 5ee9831

	case NOTIFICATION_ENTER_TREE:
	case EditorSettings::NOTIFICATION_EDITOR_SETTINGS_CHANGED: {
	uv_panner->setup((ViewPanner::ControlScheme)EDITOR_GET("editors/panning/sub_editors_panning_scheme").operator int(), ED_GET_SHORTCUT("canvas_item_editor/pan_view"), bool(EDITOR_GET("editors/panning/simple_panning")));

[AThousandShips]

LGTM

[aXu-AP]

Rebased now.

[akien-mga]

Thanks!