add sky shader

src/shaders/sky.glsl

@@ -0,0 +1,30 @@
+R"====(
+#ifdef GL_ES
+	precision lowp  int;
+	precision highp float;
+#endif
+
+varying vec4 vColor;
+varying vec2 vTexCoord;
+
+#ifdef VERTEX
+	uniform mat4 uViewProj;
+
+	attribute vec4 aCoord;
+	attribute vec4 aColor;
+	attribute vec4 aTexCoord;
+
+	void main() {
+        vColor    = aColor;
+		vTexCoord = aTexCoord.xy;
+		gl_Position = uViewProj * vec4(aCoord.xyz * 5.0, 1.0);
+		gl_Position.z = gl_Position.w;
+	}
+#else
+	uniform sampler2D sDiffuse;
+
+	void main() {
+		gl_FragColor = texture2D(sDiffuse, vTexCoord) * vColor;
+	}
+#endif
+)===="