If glslc is found, rebuild the shaders from GLSL to SPIR-V; otherwise,
we're just going to use the built files we have committed in the source
repository.
We have to work around some ordering problems here. We still
manage to keep most of the guts in modules/input/meson.build,
so it's not too ugly overall.
(The autotools build solves this with a 'make -C ../../input/modules'
inside gtk/Makefile, but that's not something we can or want to do.)
Add back dependencies on libgdk_dep and libsk_dep which are declared
dependencies. We removed this before because these declarations had
link_with: lines that dragged in the static libgdk.a and libgsk.a libs
which are linked into libgtk-4.so anyway and thus shouldn't be used
when linking internal exes/tools against libgtk-4. Remove the static
libs from the declared dependencies and have libgtk link those in
explicitly, so that the declared deps now just provide all the built
dependencies and include dirs and such for declared libgtk_dep users
such as the internal exes/tools, which want all the generated gsk/gdk/gtk
headers to exist before attempting to compile anything against the
gtk+ headers.
