Inconsistent setting of lineend in draw_key_*

[thomasp85]

How lineend is set in the draw_key functions does not seem to be standardised and e.g. differs between draw_key_polygon() and draw_key_path(). In general this is not an issue when the linetype is solid as the ends of the lines are not visible, but once the line is non-solid the lineend will be applied to each segment in the line-pattern. This can result in some dramatic visual discrepancies between what is shown in different keys and in the plot itself.

Example of difference in look between plot and legend as well as difference between polygon and path key:

ggplot() + 
  geom_segment(
    aes(x = 0, y = 0:1, xend = 1, yend = 1:0, linetype = c('a', 'b')), 
    size = 1.5, 
    lineend = 'round'
  ) + 
  geom_polygon(
    aes(x = c(0.25, 0.75, 0.5), y = c(0.12, 0.12, 0.88), linetype = 'b'), 
    fill = NA, 
    size = 1.5, 
    colour = 'red'
  ) + 
  scale_linetype_manual(values = c('solid', '11'))