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

Bug: angleproject:5174
Change-Id: If8623b1edbc8f0bac83513ea202ef49ad388101c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2489726
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>