Make Dx11 fill_buffer use an extra buffer for non 16-byte aligned fills

[Imberflur]

Fixes #3718 PR checklist:

• [x] make succeeds (on *nix)
• [ ] make reftests succeeds
• [ ] tested examples with the following backends:

[Imberflur]

Cleaned up those comments!

[kvark]

bors r=cwfitzgerald

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable