Add support for having different GBM formats for different outputs

Add support for having different GBM formats for different outputs

The gbm-format configuration option can now be specified per-output as
well as in the core config section. If it is not specified it will
default to the format specified in the core section. The
EGL_MESA_configless_context extension is required for this to work. If
this extension is available it will create a context without an
EGLConfig and then it will potentially use a different EGLConfig for
each output.

The gl-renderer interface has been changed so that it takes the EGL
attributes and visual ID in the create_output function as well as in
the create function.