According to the EXT_color_buffer_half_float spec:

"Dependencies on OES_texture_half_float

    If OES_texture_half_float is not supported, then all references to
    RGBA16F_EXT, RGB16F_EXT, RG16F_EXT, R16F_EXT, HALF_FLOAT_OES and
    half should be ignored.

    If OES_texture_half_float is supported, textures created with:

        <internalformat> = RGBA
        <format> = RGBA
        <type> = HALF_FLOAT_OES

    are renderable."

This fixes 4 viz_unittests when running with SwANGLE:
    All/GLI420ConverterPixelTest.ScaleAndConvert/0
    All/GLI420ConverterPixelTest.ScaleAndConvert/1
    GLScalerPixelTest.Example_ScaleAndExportForScreenVideoCapture
    GLScalerPixelTest.ScalesWithColorManagement

The original attempt at landing this change
(https://chromium-review.googlesource.com/c/angle/angle/+/2489726)
was reverted due to dEQP failures on the Metal backend. After some
investigation, it was decided to simply disable the 2 offending
tests for now.

Bug: angleproject:5174
Change-Id: Id1e382b72d91109e4718c729cc6c5f0a0f413d7d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2491930Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>