OpenGL2/3: Backup and restore GL_UNPACK state in UpdateTexture() (#9473)

[Turtle-PB]

Problem

The OpenGL2 and OpenGL3 backends set GL_UNPACK_ROW_LENGTH=0 and GL_UNPACK_ALIGNMENT=1 during texture creation and updates, but never restored the original values. This corrupted the calling application's pixel store state when ImGui created or updated textures mid-frame (e.g., dynamic font atlas updates).

This was flagged as a FIXME in ImGui_ImplOpenGL3_UpdateTexture():

// FIXME: Consider backing up and restoring

Fix

Backup GL_UNPACK_ROW_LENGTH and GL_UNPACK_ALIGNMENT at the start of UpdateTexture(), and restore them at the end. This matches the pattern already used for GL_TEXTURE_BINDING_2D in the same function.

OpenGL3 (imgui_impl_opengl3.cpp)

• Backup GL_UNPACK_ROW_LENGTH and GL_UNPACK_ALIGNMENT via glGetIntegerv() at function entry
• Restore them at function exit (after all WantCreate/WantUpdates/WantDestroy paths)
• Removed the FIXME comment (resolved)
• Added changelog entry

OpenGL2 (imgui_impl_opengl2.cpp)

• Same backup/restore pattern
• Added changelog entry

Testing

• The fix is guarded by #ifdef GL_UNPACK_ROW_LENGTH / #ifdef GL_UNPACK_ALIGNMENT for WebGL/ES compatibility (where these constants may not exist)
• The restore happens after all code paths (WantCreate, WantUpdates, WantDestroy), so state is always restored
• The GL_CALL macro is used for all backup/restore calls, matching the existing code style

Related Issues

• ImGui 1.92.x + OpenGL + openframeworks issue #8802 — Original change that added GL_UNPACK_ALIGNMENT=1 (but without restore)
• OpenGL2/3: Backup and restore GL_UNPACK state in UpdateTexture() (#9473) #9473 — This fix
• Supersedes the FIXME in ImGui_ImplOpenGL3_UpdateTexture()

[ocornut]

Merged as e3927dc + amends 0ff4a8a.
Original code was changing GL_UNPACK_ROW_LENGTH twice so I fixed that, and your code left the "restore to 0" so I fixed that too, removing too call. Made the code breath a little bit by bundling both #ifdef together since they were introduced in same GL version.

Thank you!