[client] renderer/egl: implement support for rendering help text

The help text is rendered in the bottom left corner on a semi-transparent
background, very similar to how the FPS text is rendered.

client/renderers/EGL/shader/help.frag

@@ -0,0 +1,11 @@
+#version 300 es
+
+in  highp vec2 uv;
+out highp vec4 color;
+
+uniform sampler2D sampler1;
+
+void main()
+{
+  color = texture(sampler1, uv);
+}