meson: bump major version

Because this is actually a rather large release, I kinda feel it's about
time to bump the major version. I still haven't decided if I want to
switch to semver or not to avoid spilling into the three digits, but oh
well. Now it's done, anyways.

meson.build

@@ -2,7 +2,7 @@ project('libplacebo', ['c', 'cpp'],
   license: 'LGPL2.1+',
   default_options: ['c_std=c99', 'cpp_std=c++11', 'warning_level=2'],
   meson_version: '>=0.51',
-  version: '2.104.0',
+  version: '3.104.0',
 )
 
 # Version number