A lot of real-world shaders start off with comments explaining the usage
or license, generating lots of "empty" passes. This simply change allows
us to skip them, which silences the warning spam and prevents us from
having to store and copy around these empty passes.
It also adds a more useful failure check: Attempting to use a user
shader that doesn't define any passes at all.
This replaces the previous TRANSFORM by WIDTH, HEIGHT and OFFSET where
WIDTH and HEIGHT are RPN expressions. This allows for more fine-grained
control over the output size, and also makes sure that overwriting
existing textures works more cleanly.
(Also add some more useful bstr functions)
This allows users to add their own near-arbitrary hooks to the vo_opengl
processing pipeline, greatly enhancing the flexibility of user shaders.
This enables, among other things, user shaders such as CrossBilateral,
SuperRes, LumaSharpen and many more.
To make parsing the user shaders easier, shaders are now loaded as
bstrs, and the hooks are set up during video reconfig instead of on
every single frame.
