optimize two calls to gvprintf with single characters

optimize two calls to gvprintf with single characters

The expression `gvprintf(j, "%c", x)` is equivalent to `gvwrite(j, &x, 1)`.
However, it seems modern compilers, even with link-time optimization enabled,
are not clever enough to see this equivalence. By unraveling the gvprintf call
to what it eventually bottoms out to, we can accelerate SVG generation.

On tests/regression_tests/large/long_chain, this drops the number of gvprintf
calls from 297008 to 165008, reducing the amount of the trace for which gvprintf
is responsible from 3.27% to 2.24%. Total executed instructions are reduced from
8160974807 to 8098098396, a speed up of ~1%.