Add possibility to create robust contexts to Gallium swrast drivers and enable it for softpipe
This series pipes through support for robustness on EGL and GXL for gallium swrast drivers. Then, as prove of concept hang detection based on a timeout is added to tgsi_exec enabling it to identify and recover endless loops (but it would also timeout on shaders with a very long per element runtime), and the related error tgsi.machine state is piped through to softpipe and used in the reset handler to initiate a context reset.