Support for adjusting button's height to fit title label.

This commit implements a new `adjusts_button_height_to_fit_title_label_` property for the `Button` class for adjusting the button's height to fit the title label's height automatically. The adjustment not only respects button's height but also vertical values defined in `title_edge_insets_`.

This commit also moves the `DeleteFramebuffer()` function from `button.cc` to `nanovg_hook.h` and renames it to `nvgDeleteFramebuffer()` so everyone can take advantage of this function.