libweston/matrix: Fix narrow compilation failure on C++

Fixes the following build failure with C++ compiler:

/include/libweston-14/libweston/matrix.h: In function ‘weston_coord
weston_coord_truncate(weston_coord)’:
/include/libweston-14/libweston/matrix.h:212:39: error: narrowing
conversion of ‘(int)in.weston_coord::x’ from ‘int’ to ‘double’
[-Werror=narrowing] 212 |         return (struct weston_coord){
	(int)in.x, (int)in.y };
      |                                       ^~~~~~~~~
/home/mvlad/install-new/include/libweston-14/libweston/matrix.h:212:50:
error: narrowing conversion of ‘(int)in.weston_coord::y’ from ‘int’ to
‘double’ [-Werror=narrowing] 212 |         return (struct weston_coord){
	(int)in.x, (int)in.y };

As we always compile with C compiler we don't see these until we use a C++ compiler.

Introduced with 1f81c082.

Signed-off-by: Marius Vlad marius.vlad@collabora.com