feat(text): optional vendored stb_truetype backend for runtime external fonts

tests/CMakeLists.txt

@@ -75,6 +75,24 @@ target_sources(plot_tests PRIVATE test_qt_pointer.cpp)
 target_sources(plot_tests PRIVATE test_gtk_input.cpp)
 target_include_directories(plot_tests PRIVATE "${CMAKE_SOURCE_DIR}/adapters")
 
+# The optional runtime-font backend brings its own cases, and they exist only when
+# its target does: the header, the class and the archive are all absent otherwise,
+# so there would be nothing to compile them against (see PLOT_BUILD_TTF_FONT in the
+# top-level CMakeLists.txt). Guarded on the target rather than on the option, like
+# the adapter cases above, so a target that exists for any reason gets its cases.
+#
+# Two paths are handed over as definitions because a case needs them and should
+# not guess: the core archive the "no stb_truetype in plot_core" case inspects
+# (a generator expression, so it is this build's own archive) and the cmake the
+# umbrella check is re-run with.
+if(TARGET plot::font_ttf)
+  target_sources(plot_tests PRIVATE test_true_type_font.cpp)
+  target_link_libraries(plot_tests PRIVATE plot::font_ttf)
+  target_compile_definitions(plot_tests PRIVATE
+    PLOT_CORE_ARCHIVE="$<TARGET_FILE:plot_core>"
+    PLOT_CMAKE_COMMAND="${CMAKE_COMMAND}")
+endif()
+
 # Same switches the library is built with: the cost tests read the counters
 # behind them (see include/plot/data/detail/{cover,merge}_count.hpp).
 target_compile_definitions(plot_tests PRIVATE PLOT_PYRAMID_COUNT_COVERS