[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);
+}

client/renderers/EGL/shader/help.vert

@@ -0,0 +1,22 @@
+#version 300 es
+
+layout(location = 0) in vec3 vertexPosition_modelspace;
+layout(location = 1) in vec2 vertexUV;
+
+uniform vec2 screen;
+uniform vec2 size;
+
+out highp vec2 uv;
+
+void main()
+{
+  gl_Position.xyz = vertexPosition_modelspace;
+  gl_Position.w   = 1.0;
+  gl_Position.xy *= screen.xy * size.xy;
+  gl_Position.x  -= 1.0 - (screen.x * size.x);
+  gl_Position.y  -= 1.0 - (screen.y * size.y);
+  gl_Position.x  += screen.x * 10.0;
+  gl_Position.y  += screen.y * 10.0;
+
+  uv = vertexUV;
+}

client/renderers/EGL/shader/help_bg.frag

@@ -0,0 +1,8 @@
+#version 300 es
+
+out highp vec4 color;
+
+void main()
+{
+  color = vec4(0.0, 0.0, 1.0, 1.0);
+}