refactor: hide backend Adapter handles

- static methods for backend adapters

vtm-playground/src/org/oscim/test/SpriteManager.java

@@ -48,7 +48,7 @@ public abstract class SpriteManager<T> {
 
 	Sprite items;
 
-	protected final Canvas mCanvas = CanvasAdapter.g.getCanvas();
+	protected final Canvas mCanvas = CanvasAdapter.newCanvas();
 	protected TextureItem mTexture;
 
 	public SpriteManager() {

vtm-playground/src/org/oscim/test/ThemeTest.java

@@ -18,7 +18,7 @@ public class ThemeTest {
 
 	public static void main(String[] args) {
 		AwtGraphics.init();
-		AssetAdapter.g = new AssetAdapter() {
+		AssetAdapter.init(new AssetAdapter() {
 			@Override
 			public InputStream openFileAsStream(String name) {
 				try {
@@ -29,7 +29,7 @@ public class ThemeTest {
 					return null;
 				}
 			}
-		};
+		});
 
 		IRenderTheme t = ThemeLoader.load(VtmThemes.DEFAULT);
 

vtm-playground/src/org/oscim/test/renderer/SymbolRenderLayer.java

@@ -33,7 +33,7 @@ public class SymbolRenderLayer extends ElementRenderer {
 		it.billboard = false;
 
 		try {
-			it.bitmap = CanvasAdapter.g.loadBitmapAsset("jar:symbols/cafe.png");
+			it.bitmap = CanvasAdapter.getBitmapAsset("jar:symbols/cafe.png");
 		} catch (Exception e) {
 			e.printStackTrace();