Shader POC: Add shader_enabled property to UIFrame (#106)

Proof of concept for shader support on UIFrame:
- Add shader and shader_enabled members to UIFrame
- Add initializeTestShader() with hardcoded wave/glow fragment shader
- Add shader_enabled Python property for toggling
- Apply shader when drawing RenderTexture sprite
- Auto-update time uniform for animated effects

Also fixes position corruption when toggling RenderTexture usage:
- Standard rendering path now uses `position` as source of truth
- Prevents box position from staying at (0,0) after texture render

Test files:
- tests/shader_poc_test.py: Visual test of 6 render variants
- tests/shader_toggle_test.py: Regression test for position bug

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/shader_toggle_test.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+"""
+Shader Toggle Test - Regression test for position corruption bug
+
+Tests that toggling shader_enabled on and off does not corrupt frame position.
+This was a bug similar to #223 where box.setPosition(0,0) during texture
+rendering was never restored when switching back to standard rendering.
+"""
+import mcrfpy
+import sys
+
+scene = mcrfpy.Scene("toggle_test")
+mcrfpy.current_scene = scene
+ui = scene.children
+
+# Background
+bg = mcrfpy.Frame(pos=(0, 0), size=(800, 600), fill_color=(20, 20, 30, 255))
+ui.append(bg)
+
+# Create test frame at a specific position
+test_frame = mcrfpy.Frame(
+    pos=(200, 200),
+    size=(150, 100),
+    fill_color=(80, 120, 180, 255),
+    outline_color=(255, 255, 255, 255),
+    outline=3.0
+)
+label = mcrfpy.Caption(text="Test Frame", pos=(10, 10), font_size=14)
+label.fill_color = (255, 255, 255, 255)
+test_frame.children.append(label)
+ui.append(test_frame)
+
+# Status display
+status = mcrfpy.Caption(text="Status: Initial", pos=(20, 20), font_size=16)
+status.fill_color = (255, 255, 100, 255)
+ui.append(status)
+
+pos_display = mcrfpy.Caption(text="", pos=(20, 50), font_size=14)
+pos_display.fill_color = (200, 200, 200, 255)
+ui.append(pos_display)
+
+instructions = mcrfpy.Caption(
+    text="Press 1: Enable shader | 2: Disable shader | Q: Quit",
+    pos=(20, 550), font_size=14
+)
+instructions.fill_color = (150, 150, 150, 255)
+ui.append(instructions)
+
+def update_display():
+    pos_display.text = f"Position: ({test_frame.x}, {test_frame.y}) | Shader: {test_frame.shader_enabled}"
+
+update_display()
+
+test_count = 0
+
+def on_key(key, state):
+    global test_count
+    if state != "start":
+        return
+
+    if key == "Num1" or key == "1":
+        test_frame.shader_enabled = True
+        status.text = "Status: Shader ENABLED"
+        update_display()
+        test_count += 1
+    elif key == "Num2" or key == "2":
+        test_frame.shader_enabled = False
+        status.text = "Status: Shader DISABLED"
+        update_display()
+        test_count += 1
+
+        # Check if position is still correct
+        if test_frame.x != 200 or test_frame.y != 200:
+            status.text = f"BUG! Position corrupted to ({test_frame.x}, {test_frame.y})"
+            status.fill_color = (255, 100, 100, 255)
+        else:
+            status.text = "Status: Shader DISABLED - Position OK!"
+            status.fill_color = (100, 255, 100, 255)
+    elif key in ("Q", "Escape"):
+        if test_frame.x == 200 and test_frame.y == 200:
+            print(f"PASS: Position remained correct after {test_count} toggles")
+        else:
+            print(f"FAIL: Position corrupted to ({test_frame.x}, {test_frame.y})")
+        sys.exit(0)
+
+scene.on_key = on_key
+
+print("Shader Toggle Test")
+print("Press 1 to enable shader, 2 to disable, Q to quit")
+print("Frame should stay at (200, 200) regardless of shader state")