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).
Fix: Backup GL_UNPACK_ROW_LENGTH and GL_UNPACK_ALIGNMENT at the
start of UpdateTexture(), and restore them at the end.
This resolves the FIXME in ImGui_ImplOpenGL3_UpdateTexture():
'FIXME: Consider backing up and restoring'
Issues: #8802 (original UNPACK_ALIGNMENT change), #9473 (this fix)
3822055e72