Use simpler shader for rect-like glyphs without rounded corners

[ianthomas23]

Rect glyphs and their relatives (Block, Quad, etc) support rounded corners via the border_radius property. The WebGL backend now has full support for these rounded corners (#13649). Because of the way GLSL shaders work (massively in parallel) the rounded corner rendering code is executed even if rounded corners are not required. This is unnecessary processing for those Rect-like glyphs that don't have rounded corners.

This PR introduces a new GLMarkerType of "round_rect" for rects with rounded corners, and keeps the use of "rect" for non-rounded corners. Actually the "rect" marker type reuses the "square" shader code, so it is not even new shader code. There is a new boolean flag on BaseMarkerGL to indicate if round corners are set and this is used to determine which GLMarkerType is used for rendering.

There are no changes at all in baseline test images.

[ianthomas23]

Note the marker_type is now determined after set_data is called as that is where _border_radius_nonzero is set.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1ce39bf) 92.55% compared to head (97eb839) 92.57%.

Additional details and impacted files

@@              Coverage Diff               @@
##           branch-3.4   #13651      +/-   ##
==============================================
+ Coverage       92.55%   92.57%   +0.02%     
==============================================
  Files             323      323              
  Lines           20525    20525              
==============================================
+ Hits            18996    19002       +6     
+ Misses           1529     1523       -6     

[mattpap]

Looks good. Is there a measurable performance difference between this and the original approach?

[ianthomas23]

Looks good. Is there a measurable performance difference between this and the original approach?

There is, just. Using the following test code on my dev machine (Quadro T1000 graphics card, ubuntu 23.10, chromium 118) and panning in small circles using this branch gives 40-45 frames per second without rounded corners, 35-40 FPS with them. branch-3.4 gives 35-40 FPS regardless of rounded corners.

from bokeh.plotting import figure, show
import numpy as np

n = 20000
rng = np.random.default_rng(12314)
x = rng.uniform(size=n)
y = rng.uniform(size=n)

backend = "webgl"
p = figure(width=800, height=800, title=f"{backend} {n}", output_backend=backend)
p.rect(
    x=x, y=y, width=0.1, height=0.1, line_width=0, fill_alpha=0.01,
    #border_radius=20,
)

show(p)

[github-actions]

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