Modernise Dependency CMake

[ptheywood]

The visualiser CMake has not been "modernised" to make heavy use of targets and aliased targets like in the main repository, but it would be a nice improvment if this were the case.

I.e. For non-native CMake projects such as SDL2, rather than just defineing variables such as SDL2_INCLUDE_DIRS we should instead define a target SDL2 which has the include dir and liker requirements set appropraitely.

Adding an alias target SDL2::SDL2 etc then makes targets appropriately namespaced.

Then, rather than specifying target_include_directories(SDL2_INCLUDE_DIRS) or similar, and teh matching target_link_libraries(SDL2_LINK_LIBRARIES) command just target_link_libraries(SDL2::SDL2) would suffice.

For the main repo, this was done as part of FLAMEGPU/FLAMEGPU2#661, with cmake/dependencies/Tinyxml2.cmake being a specific example