disp_util: allow and ignore empty strings for annotations
geom_prim: add double_pt_t and box/pt scaling that takes a double_pt_t
disp_util: improve alignment of annotation boxes (hopefully). still probably not perfect.
questionable support for global padding layers in conv_util and related functions. annotate chans in capture_feats.
test: add nopad version of alexnet prototxt template (clipped to conv5).