Capture buffer enhancements

[wqweto]

This PR allows setting capture text buffer (G->text member) initial size separately from G->buf initial size. The new optional constant is YY_TEXT_START_SIZE with a default of 1024 to preserve current behavior.

Setting YY_TEXT_START_SIZE to 0 takes care for the generated parser to not copy capture's text to G->text inside internal yyText function. This allows for copyless strategies in actions using direct access to G->buf with yycapture.begin/yycapture.end which minimized string coping/allocation.

With this change alone speed-up might be significant as greg produced parsers contain a lot of "just-in-case" calls to yyText (e.g. before expanding YY_BEGIN and YY_END macros) and usually capture strings get copied once again in actions before getting passed on the receiving callback interface (e.g. using strdup) anyway.