Backends: SDL: Added SDL_Renderer* parameter to ImGui_ImplSDL2_InitForSDLRenderer(). Use SDL_GetRendererOutputSize() instead of SDL_GL_GetDrawableSize() when bound to a SDL_Renderer. (#4927)

This is (kind of) an OpenGL-only function, which should be avoided when SDL2 isn't using OpenGL.
The only alternative that is recommended is SDL_GetRendererOutputSize, which limits this fix to the SDL_Renderer backend. Still, I think it's better than nothing.
I say that SDL_GL_GetDrawableSize is "kind of" OpenGL-only because it does technically work even when SDL2 isn't using OpenGL.
It's just that it becomes a shim to SDL_GetWindowSize, which is not suitable for high-DPI usage because it reflects the size of the window in screen coordinates, not actual pixels, so it really should be avoided when not using OpenGL.

docs/CHANGELOG.txt

@@ -86,6 +86,8 @@ Breaking Changes:
   - If calling ImGui_ImplGlfw_InitXXX with install_callbacks=true: nothing to do. is already done for you.
   - If calling ImGui_ImplGlfw_InitXXX with install_callbacks=false: you WILL NEED to register the GLFW callback
     using glfwSetCursorPosCallback() and forward it to the backend function ImGui_ImplGlfw_CursorPosCallback().
+- Backends: SDL: Added SDL_Renderer* parameter to ImGui_ImplSDL2_InitForSDLRenderer(), so backend can call
+  SDL_GetRendererOutputSize() to obtain framebuffer size valid for hi-dpi. (#4927) [@Clownacy]
 - Commented out redirecting functions/enums names that were marked obsolete in 1.69, 1.70, 1.71, 1.72 (March-July 2019)
   - ImGui::SetNextTreeNodeOpen()        -> use ImGui::SetNextItemOpen()
   - ImGui::GetContentRegionAvailWidth() -> use ImGui::GetContentRegionAvail().x