v0.3.0

Cuts per-frame and startup allocations across paint, GPU and text, and
tightens element lifetimes: elements now reuse frame nodes, so a node may
not outlive its element (breaking; see docs/el.md), icon variants and leased
glyph scratch are reused, path-space stroke outlines and frame nodes are
retained, startup font initialization allocates less, markdown follows theme
fonts, and empty carets stay clear of placeholder ink. Triple-buffered
drawable presentation is restored after the drawable-count experiment. The
font startup and stroke allocation measurements are recorded in the testing
notes.

Breaking: elements reuse frame nodes with checked lifetimes
(9c7e0c9).

PRs: #29, #30, #31, #32, #33, #34, #35, #36, #37