Add the compile-time SDL major version option and propagate it through
interface targets. Route SDL includes through C99 compatibility headers
while keeping SDL2 as the desktop and Emscripten default.
Define a dedicated `vmvar_t` type for values stored in variable pages
and propagate it through interfaces that expose those values. Keep the
type as int for now so this commit does not change runtime behavior.
This separates VM storage from ordinary int buffers and makes a later
conversion to 16-bit storage mechanically checkable.
The font module kept a global "currently selected font" set by
font_select() and implicitly used when drawing. Make it stateless by
introducing a FontSpec {type, weight, size} that font_render_text() /
font_draw_text() and gfx_drawString() take directly.
Make FontWeight a two-valued enum and normalize the 1-9 weight to it
in commandZB().
Removes font_select(), ags_setFont() and ags_setFontWithWeight().
kichikuou