Fix D3D VBOs sometimes initialised with junk data

[smoogipoo]

I noticed that every once in a while certain vertices would get incorrect data.

Furthermore this would always occur on vertices that were 0-initialised, which is always the case with exactly one vertex in FBOs since the FtB pass doesn't do anything there (depth = 0). That explains why the issues I was seeing was always when BufferedContainer + blur were involved (e.g. backgrounds, in-gameplay break overlay arrows, song select leaderboard glows).

This makes it so that we always perform one buffer update with 0-initialised data when initialising a VBO. In GLRenderer we are relying on glBufferData to both initialise the storage for us and clean out the driver/GPU buffer, so this was just missed in the port to Veldrid.

[smoogipoo]

The documentation of glBufferData lists a similar thing to D3D:

 If data is NULL, a data store of the specified size is still created, but its contents remain uninitialized and thus undefined. 

Which could explain some issues we've had in the past. I've updated this PR to include a similar fix for the GLRenderer implementation.