Add opacity slider with shader-based rendering that preserves clip mask behavior

- Added opacity slider (0-100%) in sprite options with 'affect children' checkbox
- Implemented hierarchical opacity that affects entire sprite hierarchy when enabled
- Created shader-based opacity system that doesn't interfere with clip mask functionality
- Clip masks now work with original image pixels regardless of opacity settings
- Removed shadow effects from selected sprites to prevent visual conflicts
- Updated save/load system to persist opacity properties with backward compatibility

Fixes clipping behavior issues where opacity changes previously affected clip mask detection

ui_scenes/selectedSprite/opacity.gdshader

@@ -0,0 +1,8 @@
+shader_type canvas_item;
+
+uniform float opacity : hint_range(0.0, 1.0) = 1.0;
+
+void fragment() {
+	COLOR = texture(TEXTURE, UV);
+	COLOR.a *= opacity;
+}