xtalloc: Re-prefix all ta_* symbols to xta_*

xtalloc was copied from mpv, which means that we see symbol
collisions if libplacebo is statically linked into mpv. So
use an `xta_` prefix for all the functions. In practice, this
has almost no effect on the consuming code because everything
is accessed via macros.
10 jobs for master in 1 minute and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #273518
debian aarch64
build-debian-aarch64

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macos
build-macos

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passed #273514
amd64 debian
build-ubuntu

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23.0%
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amd64 debian
build-ubuntu-static

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passed #273516
win32
build-win32

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passed #273517
win64
build-win64

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  Test
passed #273520
amd64 debian
test-ubuntu-asan

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passed #273521
amd64 debian
test-ubuntu-msan

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amd64 debian
test-ubuntu-scan

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passed #273522
amd64 debian
test-ubuntu-ubsan

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