2D streamplot tests succeed even if markers are too small to see

[ffreyer]

See for example shot_tests_51.png from v0.15. refimages:

Running the relevant code with GLMakie:

v(x::Point2{T}) where T = Point2{T}(x[2], 4 * x[1])
streamplot(v, -2..2, -2..2)

From running tests locally:

[ Info: 0.0010 == short_tests_51.png
[ Info: 0.0136 == examples2d_185_Streamplotanimation.mp4
[ Info: 0.0018 == examples2d_214_streamplot.png

We should change one of the png tests to use much larger arrow size and also adjust the 2D default to something larger without adjusting 3D (see arrows).

[ffreyer]

arrows also needs a larger markersize in the 2D test

Probably wasn't caught for the same reason?