Ray-caster: move shaders to separate files

This properly reflects the modularity of the code, and it simplifies
indentation and syntax highlighting.

app-proto/inversive-display/src/identity.vert

@@ -0,0 +1,7 @@
+#version 300 es
+
+in vec4 position;
+
+void main() {
+    gl_Position = position;
+}