PERF: Text rendering speedups - #32120

PR summary

Inspired by #32064, I dug into text rendering to try and see what we could speed up. Through a combination of caching, lazy loading, and combining runs of glyphs into a single rasterization pass, total figure draw time on my simple demo script below is sped up by 2.4x. This is separate from (and stacks on top of) the improvements in #32064. Beyond the immediate rendering, text is also handled in figure/subplot layout, so there are speedups across the entire call tree.

This is a big diff and stacked PRs aren't available yet, so is probably easiest to review by commit. Each is self contained, and here is a table describing them:

Before (4.03 sec draw):

After (1.86 sec draw):

Benchmark script:

import statistics
import time
import matplotlib.pyplot as plt

fig, ax = plt.subplots(figsize=(8, 6), dpi=100)

# Simple plot with several types of text
ax.plot([1, 2, 3], [1, 4, 2], label="series one")
ax.set_title("Rendering a title with plenty of glyphs")
ax.set_xlabel("x axis label")
ax.set_ylabel("y axis label")
ax.legend()

for i, text in enumerate(["one", "two", "three", "four"]):
    ax.text(1, 1 + i * 0.5, "annotation" + text)
ax.text(2, 3, "rotated annotation", rotation=45)

fig.canvas.draw()  # warm the cache

times = []
for _ in range(50):
    start = time.process_time()
    fig.canvas.draw()
    times.append(time.process_time() - start)

print(f"draw: median={statistics.median(times) * 1e3:.2f} ms  "
      f"min={min(times) * 1e3:.2f} ms")

AI Disclosure

Lots of help from claude on the first pass for this one. I spent several hours reviewing and polishing its draft, and am confident in each of the changes. What I am less confident on is potential corner cases of the text rendering pipeline that I'm unfamiliar with, so would appreciate @QuLogic to take a look through this when he has the time.

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines