glcolorconvert: Fix external-oes shader

The #extention must come before 'precision highp float;'.

Closes: #650
15 jobs for 1.16 in 74 minutes and 59 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #498495
gst indent

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passed #498494
manifest

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  Build
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build cerbero android universal

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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passed #498505
gst-ios-12.2
build cerbero ios universal

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passed #498504
gst-macos-10.14
build cerbero macos x86_64

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build fedora x86_64

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passed #498497
build nodebug fedora x86_64

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build static fedora x86_64

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passed #498499
build static nodebug fedora x86_64

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  Test
passed #498506
check fedora

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passed #498507
integration testsuites fedora

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  Integrate
passed #498508
android universal examples

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