testsuite: Update memorytexture test for TextureDownloader

testsuite: Update memorytexture test for TextureDownloader

There is no longer a need to use gdk_texture_download() and force
conversion to ARGB8 format. We can download the pixels in the original
format again.

That way we avoid testing the conversion code and avoid having to deal
with differences in representable colors.

However, some formats do do conversions, so we allow pixel comparisons
to be accurate (requires 16bit comparison accuracy) or inaccurate (we
only care about 8bit).
Note that for the default RGBA formats, this is identical and means they
need to be bit-exact the same, no matter what.
But the higher bit depth formats may be more different - floating point
can even have different values with high accuracy (the float mantissa is
23 bit, we only care about 16).