Investigate and decide on what we do for i.MX6 (Linux)

Things run on i.MX6 (testing with a Boundary Devices BD-SL and the Boot2Qt 5.11 image) using OpenGL ES 3.0, but there are some issues with the default material and some "FBO incomplete" messages when trying to use shadows.

1.1 used to fall back to the crippled OpenGL ES 2.0 path (so I am told), which is an option for 2.0 as well. However, we should first investigate what is exactly happening when using ES 3.0 path as in theory there is no reason for runtime 2.0 to not work on the i.MX6 using the better featured rendering path.

conclusion: we will force the gles2 path and limit the number of lights per layer to 2.