Bad Vulkan performance due to weak index/vertex buffer management

[d-s-h]

Describe the bug
I noticed that the Vulkan version of bgfx has a performance bottleneck comparing to D3D11 in my app.
During a short investigation I found out the same issue is applicable even for the bgfx helloworld example. On my notebook the frame time can jump up to 25ms using Vulkan, while it's stable less than 1ms when using D3D11.
I identified that the bottleneck goes to the following profile scope:

BGFX_PROFILER_SCOPE("bgfx/Update transient index buffer", kColorResource);

I'll add my dirty fix with some hardcode for reference, but it should be remade.

To Reproduce
Steps to reproduce the behavior:

1. Run HelloWorld example using Vulkan.
2. See the frametime. It's inadequate high and not stable.

Expected behavior
Smooth as butter and fast as light.

Screenshots

Additional context
Checked on Windows 11 and Android 13 platforms.

[d-s-h]

Here is my dirty fix:
Fix Vulkan perf bottleneck with staging buffers. There is a limit the staging scratch buffer to 10Mb.