[docs] Include uncompressed shader source in .cpp comments

This is meant to
(1) Make it easier for new developers to find the source
(2) Make it easier to look at shader diffs when the GL JS pin changes

src/mbgl/shaders/debug.cpp

@@ -10,5 +10,27 @@ const char* debug::name = "debug";
 const char* debug::vertexSource = source() + 17919;
 const char* debug::fragmentSource = source() + 18031;
 
+// Uncompressed source of debug.vertex.glsl:
+/*
+attribute vec2 a_pos;
+
+uniform mat4 u_matrix;
+
+void main() {
+    gl_Position = u_matrix * vec4(a_pos, 0, 1);
+}
+
+*/
+
+// Uncompressed source of debug.fragment.glsl:
+/*
+uniform highp vec4 u_color;
+
+void main() {
+    gl_FragColor = u_color;
+}
+
+*/
+
 } // namespace shaders
 } // namespace mbgl