html/template: grow srcset buffer in proportion to need

html/template: grow srcset buffer in proportion to need

In particular, avoid exponential memory usage from growing it in
proportion to its current size.

Fixes #24731

Change-Id: I277d2fbac2ef7b00ae4b83d6d1dcd7f2e630a5cd
Reviewed-on: https://go-review.googlesource.com/105155
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>