d3d12: When flushing+finishing, enqueue a new fence

Fixes #36, though unfortunately not enough to get Minecraft to run. Now we crash in TexSubImage2D due to running out of memory (7.8GB):

C  [opengl32.dll+0x84bcea]  d3d12_bo_get_base+0xa
C  [opengl32.dll+0x84e9fe]  d3d12_bufmgr_create_buffer+0x16e
C  [opengl32.dll+0xf4aa76]  pb_cache_manager_create_buffer+0x86
C  [opengl32.dll+0x84bf5c]  d3d12_resource_create+0x12c
C  [opengl32.dll+0x84df41]  pipe_buffer_create+0x61
C  [opengl32.dll+0x84c5fd]  d3d12_transfer_map+0x12d
C  [opengl32.dll+0x6113dd]  u_default_texture_subdata+0x7d
C  [opengl32.dll+0x105a43]  st_TexSubImage+0x2f3
C  [opengl32.dll+0x187122]  texture_sub_image+0x162
C  [opengl32.dll+0x185c20]  texsubimage_err+0x250
C  [opengl32.dll+0x17c6df]  _mesa_TexSubImage2D+0x9f