feat(svm): draw the primitives, an arc as an arc

[andiwand]

🤖 Generated with Claude Code

Stage 6 of #772, stacked on #787 → #786 → #785 → #784 → #779 — review those
first; this branch's base is feat/svm-clipping.

LINE, ELLIPSE, ROUNDRECT, ARC, PIE, CHORD, POINT and PIXEL were
all skipped. Seven of the eight occur nowhere in the 1125 harvested metafiles —
but LINE occurs 47 times in two of them, and it turns out to be what rules
a table: odt/style+svm-various-2.odt and
odt/style+charset+svm-various-1.odt had their tables come out as floating
numbers with no grid at all. Those two pages are the only reference output this
branch moves.

An arc is svg's A, not a polygon

svgwriter.cxx turns an arc into a polygon of up to 256 points because it has
nothing better. svg has A.

The geometry is ImplPolygon's (tools/source/generic/poly.cxx): centre and
radii from the rectangle, each ray's ellipse parameter
atan2(radius_x·sin θ, radius_y·cos θ) over the angle θ to the point, and vcl
sweeps the mathematically positive way round — counter-clockwise on screen,
which is svg's sweep flag 0. A start ray equal to the end ray is the whole
ellipse rather than nothing (#i73608#), and since svg draws nothing for an
arc that ends where it starts, every arc goes out as its two halves.

Checked against LibreOffice on a metafile written for the purpose:

	LibreOffice	this branch
arc start	(441, 544)	(441, 544)
arc end	(100, 650)	(100, 650)
pie	closes through (300, 650), the centre	same
chord	closes back to (441, 544)	same
ellipse	cx=300 cy=250 rx=200 ry=150	same
round rect	rx=60 ry=40	same

A point is one device pixel

…which no length in the drawing can be, since the drawing scales. Both POINT
and PIXEL go out as a dot of the hairline's own width — a degenerate path
with a round cap — rather than the zero-length <line> svgwriter.cxx writes,
which draws nothing at all. A PIXEL carries its own colour, and nothing else
draws with it.

Verification

5 new tests (34 in the svm suite), the arc one asserting the exact path against
the numbers LibreOffice produced above. Reference output regenerated (2 private
files, no public ones) and the pin advanced here.