Revert changing default value of ImTextureID_Invalid to -1. Back to 0. (#9295, #9310, #9293, #8745, #8465, #7090)

Reverts 0db591935f

imgui.cpp

@@ -395,10 +395,6 @@ IMPLEMENTING SUPPORT for ImGuiBackendFlags_RendererHasTextures:
  When you are not sure about an old symbol or function name, try using the Search/Find function of your IDE to look for comments or references in all imgui files.
  You can read releases logs https://github.com/ocornut/imgui/releases for more details.
 
- - 2026/03/12 (1.92.7) - Changed default ImTextureID_Invalid to -1 instead of 0 if not #define-d. (#9293, #8745, #8465, #7090)
-                         It seems like a better default since it will work with backends storing indices or memory offsets inside ImTextureID, where 0 might be a valid value.
-                         If this is causing problem with e.g. your custom ImTextureID definition, you can add '#define ImTextureID_Invalid 0' to your imconfig.h + PLEASE report this to GitHub.
-                         If you have hard-coded e.g. 'if (tex_id == 0)' checks they should be updated. e.g. OpenGL2, OpenGL3 and SDLRenderer3 backends incorrectly had 'IM_ASSERT(tex->TexID == 0)' lines which were replaced with 'IM_ASSERT(tex->TexID == ImTextureID_Invalid)'. (#9295)
  - 2026/02/26 (1.92.7) - Separator: fixed a legacy quirk where Separator() was submitting a zero-height item for layout purpose, even though it draws a 1-pixel separator.
                          The fix could affect code e.g. computing height from multiple widgets in order to allocate vertical space for a footer or multi-line status bar. (#2657, #9263)
                          The "Console" example had such a bug: