Fix assertion failure when ImFontAtlasRectEntry::Generation overflows

[cfillion]

Letting the following snippet run for a while (~40s @ 240FPS) triggers an assertion failure due to ImFontAtlasRectEntry::Generation overflowing:

ImGui::PushFont(nullptr, rand() % 512);
ImGui::Text("Lorem ipsum dolor sit amet");
ImGui::PopFont();

../../imgui_draw.cpp:4263: ImFontAtlasRectId ImFontAtlasPackAllocRectEntry(ImFontAtlas*, int): Assertion `index_entry->IsUsed == false && index_entry->Generation > 0' failed.

The assertion in ImFontAtlasRectId_Make was also off by one (takes ~1.5m @ 240FPS to trigger):

../../imgui_internal.h:3744: ImFontAtlasRectId ImFontAtlasRectId_Make(int, int): Assertion `index_idx < (0x000FFFFF) && gen_idx < ((0x3FF00000) >> (20))' failed.

EDIT: Tightened the assert in ImFontAtlasRectId_Make to catch would-be negative TargetIndex values (<0 and 0x80000-0xFFFFF).

[ocornut]

Thank you!
This is merged as a309d2d
I made two small changes:

• changed the while (++index_entry->Generation == 0); into an additional if so it is clearer to read.
• made ImFontAtlasRectId_GetGeneration() also return unsigned int.

FYI the ImFontAtlasRectId_IndexMask_ is rather large, left-over 2^19 is ~512 KB, so we could decide move some bits to GenerationMask_ if desired. GenerationMask_ was initially aimed to allow GetCustomRect()/RemoveCustomRect() to safely fail on an incorrect identifier, but we started relying on that more (see the bool add_and_draw = (atlas->GetCustomRect(builder->PackIdMouseCursors, &r) == false);) line so we are kind of at the mercy of a possible failure if both Generation and Index are identical after a full wraparound of the generation index. I'm not really worrying about it, similar to CRC32 I think the collision may easily be manufactured but will not happen in the real world, and we are catering for the later.

[ocornut]

FYI you can change to the % 512 to e.g. % 128 to triggers this faster.
(This also showcase the fact that we ideally should have better configurable heuristic to grow the atlas texture.)