Fix unintended switch fall through in DShowSource

This shouldn't have any visible impact except for slightly lower memory usage
for retro deinterlacing
Add CreatePixelShaderFromFileAsync API call

This should cause less problems than the shader cache, though unfortunately it
makes the client code slightly more complicated

CreateVertexShaderFromFileAsync should not be necessary for OBS vertex shaders
Remove shader cache

This should eliminate all problems users had with corrupted shader cache files

API users who need non-blocking compilation of shaders are encouraged to use the
new async API
Use async shader compilation for deinterlacing shaders in DShowSource