android: Fix for android OpenGL ES failures

Android devices that do not support high precision were failing
on an OpenGL error due to no precision specifiction. The result
was no GUI display, just a black screen.

mythtv/libs/libmythui/mythrender_opengl2es.cpp

@@ -20,6 +20,8 @@ void MythRenderOpenGL2ES::InitProcs()
     // GLSL ES precision qualifiers
     m_qualifiers = "#ifdef GL_FRAGMENT_PRECISION_HIGH\n"
                    "precision highp float;\n"
+                   "#else\n"
+                   "precision mediump float;\n"
                    "#endif\n";
 
     // Default OpenGL ES 2.0