[BUG] 'dashed' line style not working properly when using webgl

[TheoMathurin]

Bokeh 2.2.3
Python 3.8.5
Ubuntu 18.04
Notebook or Bokeh server
Latest Firefox or Chromium

I've tried webgl rendering to speed up my scatter plots, with great success. However I use a line in these plots and noticed that all possible values for the line_dash keyword argument work except 'dashed', although the legend item seems to adapt just fine.

This is a rather minor issue but I figured it should be noted somewhere.

Minimal code:

from bokeh.plotting import figure
from bokeh.io import curdoc
from bokeh.layouts import gridplot

p1 = figure(title='No webgl')
p1.line([0, 1, 2, 3], [2, 5, 1, 6], line_dash='dashed', line_width=3, legend_label='Line1')

p2 = figure(title='webgl', output_backend='webgl')
p2.line([0, 1, 2, 3], [2, 5, 1, 6], line_dash='dashed', line_width=3, legend_label='Line1')

curdoc().add_root(gridplot([p1, p2], ncols=2, merge_tools=True))

[bryevdv]

although the legend item seems to adapt just fine.

IIRC legends are not rendered using webgl regardless of the main plot.