Fix highdpi screens on SDL examples with viewports

[rasky]

(Dup of #2284, done over the docking branch now, as requested by @ocornut)

imgui_impl_opengl3.cpp and imgui_impl_opengl2.cpp currently have a bug in the docking branch when used with a HighDPI configuration. I discovered this bug while testing ImGui in an application that turns on HighDPI (not to improve rendering of imgui itself, but rather the application itself over which ImGui is drawn).

To reproduce this bug, you can use any OpenGL-based example. I personally use SDL so it's possible to trigger this by modifying example_sdl_opengl3/main.cpp to add SDL_WINDOW_ALLOWHIGHDPI when calling SDL_CreateWindow(). After having added this flag, running the example immediately produces a broken output like this:

The bug is in ImGui_ImplOpenGL3_RenderDrawData: the draw_data->DisplayPos variable is not scaled by the current framebuffer scale.

This PR fixes this bug for both OpenGL2 and OpenGL3 implementation. Moreover, it activates HighDPI rendering on SDL examples. This is not strictly required to fix any bug, but I think it's a good idea to keep it active because it increases the amount of code being tested by running the examples, and is thus easier to uncover hidden bugs with highdpi scaling.

[ocornut]

Hello @rasky,

Both of your change have been integrated through various commits in made yesterday.

For io.DisplayFramebufferScale I have researched it a little more and concluded this should be an information provided in ImDrawData. This also makes the render function independent of ImGuiIO fields. I made the clip rectangle offset/scale code explicit and not relying on ScaleClipRects(). This function being destructive made it impossible to call render multiple times (e.g. for VR) so it may be sane to just obsolete it, but I also need old/copied bindings to still work. We could have core imgui itself transform the ClipRect but I am not yet confident this would satisfy every use case. For now I'm doing the transform in the bindings that are OSX compatible.

See a79785c

For SDL_WINDOW_ALLOWHIGHDPI: I added it in the main.cpp files also in master branch, and in viewport/docking I am inheriting/copying the flag from the main viewport.

Thanks for the help!

[rasky]

Thanks I'll test them.