Fix brightness and ghost effects

The renderer now uses premultiplied alpha in the frame buffer, though
textures are still stored without premultiplied alpha. This makes it
easier to get the desired results from the browser's canvas compositing
step, and might be nicer for mobile hardware depending on which parts of
the Internet you believe.

src/Drawable.js

@@ -95,10 +95,10 @@ Drawable._effectConverter = {
         return Math.max(1, Math.min(x, 512));
     },
     brightness: function(x) {
-        return Math.max(-100, Math.min(x, 100));
+        return Math.max(-100, Math.min(x, 100)) / 100;
     },
     ghost: function(x) {
-        return Math.max(0, Math.min(x, 100));
+        return 1 - Math.max(0, Math.min(x, 100)) / 100;
     }
 };
 
@@ -312,6 +312,8 @@ Drawable.prototype._setSkinCore = function (source, costumeResolution) {
 
     var options = {
         auto: true,
+        mag: this._gl.NEAREST,
+        min: this._gl.LINEAR, // TODO: mipmaps
         src: source
     };
     var willCallCallback = typeof source == 'string';

src/index.js

@@ -29,7 +29,7 @@ function RenderWebGL(
     // TODO: remove?
     twgl.setDefaults({crossOrigin: true});
 
-    this._gl = twgl.getWebGLContext(canvas);
+    this._gl = twgl.getWebGLContext(canvas, {alpha: false});
     this._drawables = [];
     this._projection = twgl.m4.identity();
 
@@ -105,8 +105,8 @@ RenderWebGL.prototype.draw = function () {
     gl.clear(gl.COLOR_BUFFER_BIT);
 
     gl.enable(gl.BLEND);
-    gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
-    
+    gl.blendFuncSeparate(gl.ONE, gl.ONE_MINUS_SRC_ALPHA, gl.ZERO, gl.ONE);
+
     var currentShader = null;
 
     var numDrawables = this._drawables.length;

src/shaders/sprite.frag

@@ -108,9 +108,9 @@ void main()
 
 	#ifdef ENABLE_pixelate
 	{
-	    // TODO: understand why this padding helps clean up "pixel" edges
-	    const vec2 pixelPadding = vec2(0.125, 0.125);
-	    vec2 pixelTexelSize = u_skinSize * u_scale / u_pixelate;
+		// TODO: understand why this padding helps clean up "pixel" edges
+		const vec2 pixelPadding = vec2(0.125, 0.125);
+		vec2 pixelTexelSize = u_skinSize * u_scale / u_pixelate;
 		texcoord0 = (floor(texcoord0 * pixelTexelSize + pixelPadding)) / pixelTexelSize;
 	}
 	#endif // ENABLE_pixelate
@@ -186,4 +186,7 @@ void main()
 	#ifdef ENABLE_ghost
 	gl_FragColor.a *= u_ghost;
 	#endif // ENABLE_ghost
+
+	// WebGL defaults to premultiplied alpha
+	gl_FragColor.rgb *= gl_FragColor.a;
 }