Add OpenGL 4.6 backend for NanoVG

This commit introduces a new header file `nanovg_gl46.h` that implements
the NanoVG rendering context using OpenGL 4.6. The new backend utilizes
Direct State Access (DSA) functions, immutable texture storage, and
explicit layout qualifiers. It includes definitions for creating and
deleting the rendering context, as well as functions for handling images
with OpenGL 4.6.

CMakeLists.txt

@@ -57,9 +57,9 @@ if(stb_ADDED)
     add_library(stb INTERFACE)
     target_include_directories(stb INTERFACE ${stb_SOURCE_DIR})
 endif()
-
 # ── Local dependencies ───────────────────────────────────────────────────────
 add_subdirectory(external/glad)
+add_subdirectory(external/nanovg)
 
 find_package(OpenGL REQUIRED)
 find_package(X11 REQUIRED)
@@ -90,6 +90,7 @@ target_link_libraries(${PROJECT_NAME}
         Lua::Library
         sol2
         stb
+        nanovg
         glad
         PkgConfig::GST
 )