An authored layer's effect buffer spanned only its painted content — the glyph
bounding box for text, the drawn bounds for shapes — while the size the stack
measures from (effect_bounds) is the whole comp-sized layer rectangle. Tiling
and displacement effects computed correct output for pixels outside the painted
rectangle and then wrote them into the buffer that was too small, so CC Tiler
repeated only within the text box and CC Smear lost its liquid trail at the
glyph edge (#658). Buf::cover now grows the stack's input to the layer's
rectangle with transparent margins — what a sized source's buffer already is —
applied by run_effect_stack whenever an enabled video effect will run, and
natively on the GPU by reallocating and copying the texture. The placement and
the sized-source paths are unchanged; two regression tests (CC Tiler spans the
region, CC Smear trails past the text bounds) fail without the growth.
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