autotools: correctly build canvas-related code

spice client and spice server shares code from
common/{gdi,gl,sw}_canvas.[ch]. However, while most of the code is
shared, the server code wants a canvas compiled with
SW_CANVAS_IMAGE_CACHE defined while the client code wants a canvas
compiled with SW_CANVAS_CACHE.

The initial autotools refactoring didn't take that into account,
this is now fixed by this commit. After this commit, the canvas
files from common/ are no longer compiled as part of the
libspice-common.la convenience library. Instead, there are "proxy"
canvas source files in client/ and server/ which #include the
appropriate C files after defining the relevant #define for the
binary that is being built.

To prevent misuse of the canvas c files and headers in common/,
SPICE_CANVAS_INTERNAL must be set when including the canvas headers
from common/ or when building the c files from common/ otherwise
the build will error out.

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