Shader demos need modification for Raspberry Pi

I'm using OpenFrameworks v0.9.3 on a Raspberry Pi 3, and the shader examples don't work out of the box, failing with the following message:

[ error ] ofShader: sorry, it looks like you can't run 'ARB_shader_objects'
[ error ] ofShader: please check the capabilites of your graphics card: http://www.ozone3d.net/gpu_caps_viewer
[ error ] ofShader: setupShaderFromSource(): failed creating GL_VERTEX_SHADER shader
[ error ] ofShader: sorry, it looks like you can't run 'ARB_shader_objects'
[ error ] ofShader: please check the capabilites of your graphics card: http://www.ozone3d.net/gpu_caps_viewer
[ error ] ofShader: setupShaderFromSource(): failed creating GL_FRAGMENT_SHADER shader
[ error ] ofShader: linkProgram(): trying to link GLSL program, but no shaders created yet

However, this is easily resolved by modifying the following two lines of main.cpp

        ofGLWindowSettings settings;
        settings.setGLVersion(3,2);

to

        ofGLESWindowSettings settings;
        settings.glesVersion = 2;

Should be an easy enough fix, with appropriate $#ifdefs. I'm happy to try my hand at a PR.

Start with the shader examples' main.cpp and inspect how the window settings are selected for Raspberry Pi. Check the platform-specific configuration and verify the shader demos on a Raspberry Pi 3; done means they create and link their shaders without the reported ARB_shader_objects errors.