various changes:

- renamed example shaders to .kage
- added --map-* options to map builtin uniforms to custom names
- added ebitengine sample shader with mapping (run `make shader-ebiten`)
- fixed bug: shaders with 0 images are allowed now

example/destruct.kage

@@ -0,0 +1,37 @@
+// Copyright 2020 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build ignore
+
+//kage:unit pixels
+
+package main
+
+var Flag int
+var Slider float
+var Time float
+var Mouse vec2
+
+func Fragment(_ vec4, texCoord vec2, _ vec4) vec4 {
+	wallpx := imageSrc0At(texCoord) // A pixel from the wall tile
+	mask := imageSrc1At(texCoord)   // A pixel from the damage mask image
+
+	if Flag == 1 && (wallpx.a != 0.0 && mask.a != 0.0) {
+		alpha := clamp(Slider+(1.0-mask.a), 0.0, 1.0)
+		// Create a darker pixel if it's inside a damage mask.
+		return vec4(wallpx.r*alpha, wallpx.g*alpha, wallpx.b*alpha, wallpx.a)
+	}
+
+	return wallpx // Otherwise, leave a pixel color as is
+}