[x265] Add multilib feature with runtime bit-depth switching

[Shen (QShen3)]

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[Billy O'Neal (BillyONeal)]

Does upstream expect playing these kinds of merge library games?

[Shen (QShen3)]

I really don't like the idea that build the library 3 times and merge the results into one library. But this looks like the only way to produce multi-bit-depth library according to the design.
build implications from x265 doc

By default libx265 will place all of its internal C++ classes and functions within an x265 namespace and export all of the C functions documented in this file. Obviously this prevents 8bit and 10bit builds of libx265 from being statically linked into a single binary, all of those symbols would collide.
However, if you set the EXPORT_C_API cmake option to OFF then libx265 will use a bit-depth specific namespace and prefix for its assembly functions (x265_8bit, x265_10bit or x265_12bit) and export no C functions.
In this way you can build one or more libx265 libraries without any exported C interface and link them into a libx265 build that does export a C interface. The build which exported the C functions becomes the default bit depth for the combined library, and the other bit depths are available via the bit-depth introspection methods.

I could remove 10bit and 12bit features and the port will be looked like this:

• x265: default 8bit build
• x265[multilib]: 8bit + 10bit + 12bit

Or, just follow recommendation from the doc and build the multilib by default.

We recommend that packagers distribute a single combined shared/static library build which includes all the bit depth libraries linked together. See the multilib scripts in our build/ subdirectories for examples of how to affect these combined library builds. It is the packager’s discretion which bit-depth exports the public C functions and thus becomes the default bit-depth for the combined library.

Billy O'Neal (@BillyONeal) Which do you think is better

[Shen (QShen3)]

A patch is created to move all the logic into x265's CMakeLists.txt so that the portfile.cmake is clean now.

When ENABLE_MULTILIB is set to ON, 10 bit and 12 bit static build target will be defined and do the merge in post build step.

[Billy O'Neal (BillyONeal)]

Given that this appears to implement what upstream has explicitly asked packagers to do have you submitted this upstream? I suspect that their comments will make whether this should be a feature clearer.

[Shen (QShen3)]

Multicorewareinc/x265#955 A PR is created for upstream

[Billy O'Neal (BillyONeal)]

Drafting due to check failures.