Bug when using grayscale display_method

[fjperezgrau]

When selecting grayscale display_method, the output image is constantly flashing, most likely due to collision of the image publishers in both images and events callbacks. In renderer.cpp, the flag used_last_image_ is never set to true in the case of grayscale, so lines 150-151 could instead be this, in a similar way as it is done in the case of red-blue display_method.

[guillermogb]

Thanks. Addressed here: https://github.com/uzh-rpg/rpg_dvs_ros/pull/74