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

@@ -20,7 +20,7 @@ package main
 
 var Flag int
 var Slider float
-var Time int
+var Time float
 var Mouse vec2
 
 func Fragment(_ vec4, texCoord vec2, _ vec4) vec4 {

example/ebiten.kage

@@ -0,0 +1,33 @@
+// 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 Time float
+var Cursor vec2
+
+func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
+	pos := (dstPos.xy - imageDstOrigin()) / imageDstSize()
+	pos += Cursor / imageDstSize() / 4
+	clr := 0.0
+	clr += sin(pos.x*cos(Time/15)*80) + cos(pos.y*cos(Time/15)*10)
+	clr += sin(pos.y*sin(Time/10)*40) + cos(pos.x*sin(Time/25)*40)
+	clr += sin(pos.x*sin(Time/5)*10) + sin(pos.y*sin(Time/35)*80)
+	clr *= sin(Time/10) * 0.5
+	return vec4(clr, clr*0.5, sin(clr+Time/3)*0.75, 1)
+}