Parse SVG arc paths with packed large-arc/sweep flags (fixes #129)

[gaoflow]

Fixes #129.

The bug

parse_path() raises IndexError on valid SVG arc commands whose large-arc-flag/sweep-flag are written without a separator before the following number:

>>> from svgpathtools import parse_path
>>> parse_path("M 0 0 A 5 5 0 0110 0")
IndexError: pop from empty list
>>> parse_path("M 0 0 A 5 5 0 0 1 10 0")     # the spaced-out form works
Path(Arc(start=0j, radius=(5+5j), rotation=0.0, large_arc=False, sweep=True, end=(10+0j)))

Per the SVG path grammar, each flag is a single "0"/"1" token that may be immediately followed by the next number with no separator, so 0110 0 means flag 0, flag 1, number 10, number 0. Every browser and librsvg accept this, and SVGO emits it to save bytes — which is exactly the real-world path in #129 (the simple-icons "emlakjet" icon, …a3.543 3.543 0 00-1.267…).

Root cause

_tokenize_path tokenizes every command's arguments with FLOAT_RE.findall:

for token in FLOAT_RE.findall(x):
    yield token

FLOAT_RE is greedy and command-agnostic, so for an arc it reads the packed flag pair 0110 as the single number 110. The arc branch then pops seven values (rx, ry, rotation, large_arc, sweep, x, y) but only five tokens exist, so elements.pop() empties the list → IndexError at path.py:3361.

The fix

Tokenize arc arguments flag-aware: the two flag fields are read one "0"/"1" character at a time, while every other field stays a normal (greedy) number. The resulting token stream for a packed arc is byte-identical to its spaced-out form, so the existing pop-based parser is unchanged — only the tokenizer learned that arc flags are single characters. Non-arc commands keep using FLOAT_RE.findall exactly as before.

This mirrors the upstream svg.path library (from which this parser was originally copied), which already tokenizes arc flags this way.

Tests

test_arc_flag_packing asserts that six packed forms (absolute and relative, a negative coordinate directly after a flag, and a two-arc group with the second packed) parse identically to their spaced-out equivalents, that the flags carry through to the resulting Arc, and that the real #129 simple-icons path parses instead of raising. The new test fails on master with the original IndexError; the full suite (102 tests) passes with the fix.