SDL3 GPU - Reuse transfer buffers

[TheMode]

Quickly tested on the docking branch, hopefully not breaking anything.

There is another transfer buffer de/allocation in ImGui_ImplSDLGPU3_UpdateTexture but it doesn't seem relevant for my existing app. Should perhaps be replaced as well.

Close #8534

[ocornut]

Oh wow, I literally just pushed my own code to solve this and noticed your PR afterwards.
I'll compare them now..

[TheMode]

What a timing :)
Well yours seem a bit better, so here we go

[ocornut]

So, I pushed b678659 just as you pushed #8535. The only difference seems that I moved that code into the same function after the main buffer creation, which seems sensible as the values are synchronized. So it's a little less code.
Sorry that our streams crossed with such a tight timing, I didn't realize you were aiming at a PR.
Thanks again!

[ocornut]

There is another transfer buffer de/allocation in ImGui_ImplSDLGPU3_UpdateTexture but it doesn't seem relevant for my existing app. Should perhaps be replaced as well.

This is particularly important to the dynamic_fonts branch expected to be 1.92 so I'll work on caching this one too. Thanks for the the head up.

[ocornut]

This is the equivalent change for dynamic_fonts branch: 9802c61