The build was failing with an undefined reference to `_caca_alloc2d`.
This was because the function was internal to libcaca but used by
the `cacaview` utility.

This change makes `_caca_alloc2d` a public function `caca_alloc2d`
by moving its declaration to the public header `caca.h`. All call
sites have been updated to use the new function name.

This fixes the build and allows the Python bindings to be used.

Patch courtesy of Jakub Bogusz.

These were all caused by sprintf() appending a null char that was not actually used.
Fixes CVE-2021-30498 and CVE-2021-30499.

Fixes: #52 (CVE-2021-3410)

Fixes: #38 (CVE-2018-20546)
Fixes: #39 (CVE-2018-20547)

Fixes: #36 (CVE-2018-20544)

The slang internal auto-detection of utf8 is only executed by a call to
SLutf8_enable(-1).

- Only affect slang/ncurses.

The Xlib and ncurses libraries query the LC_CTYPE locale value to guess
the usable character set. If the calling program did not call setlocale(),
that character set will be severely limited. Extensive Unicode support
is a reasonable libcaca user expectation.

The locale is restored as soon as possible, once the window or terminal
have been initialised. Unfortunately, the effect of setlocale() is process-
wide, and may affect other threads. This is now documented.

Note also that both Xlib and ncurses ignore the effects of uselocale()
which would have been the thread-safe solution to this problem.

This fixes the path to source files in some unit tests and allows
to get rid of a few duplicate lines.

- Save and correctly restore console input mode, ie. do not
   change settings of the original console.
 - Fix hiding of cursor.
 - Free console again if the driver actually created a new one.
 - Default canvas size is current window size.
 - Base calculation on current font.
 - Properly handle press and release events of up to 5 mouse buttons.
 - Map right mouse button to button #3. This is a change in behaviour,
   which is desirable for cross-platform codes.
 - Generate two click events for double mouse clicks.
 - Mouse wheel support, creates button press events. Button numbers
   are identical to X11 mappings.
 - Handle window buffer resize events.

unit test for the problem. Bug found by Bastian Märkisch <bmaerkisch@web.de>.

outside the canvas.