Expose USE_D3D11_STAGING_BUFFER for use from build scripts.

[jdryg]

I was wondering if it's possible to put USE_D3D11_STAGING_BUFFER inside an #ifndef/#endif block so I can define it from the build script instead of editing bgfx files.

Thanks in advance.

[bkaradzic]

I can add that.

[bkaradzic]

Forgot about it... :) Here: 8ec4b71