move shaders into .glsl files and add #include feature

data/shaders/texture-alpha.glsl

@@ -0,0 +1,24 @@
+[[vertex]]
+uniform mat4 mvp;
+in vec4 pos;
+in vec2 uvs;
+out vec2 uv;
+void main()
+{
+    uv = uvs;
+    gl_Position = mvp * vec4(pos.xyz, 1.0);
+};
+
+[[fragment]]
+uniform sampler2D tex;
+uniform mediump float alpha;
+uniform bool highlight;
+in mediump vec2 uv;
+out mediump vec4 frag;
+void main()
+{
+    mediump vec4 c = texture(tex, uv);
+    frag = highlight ? 
+        vec4(clamp(vec3(0.3) + c.rgb, vec3(0.0), vec3(1.0)), c.a) : 
+        c * vec4(1.0, 1.0, 1.0, alpha);
+};