Fix texture conversion in the GL backend

[goodartistscopy]

The line as currently written disallows conversions (except swizzle) because convert can't be true if compressed isn't.

[bkaradzic]

Specify which format -> format conversion fails?

Flipping logic would disallow decompressing textures.

[goodartistscopy]

Conversion is fine but it's never called, because compressed == false prevents convert == true.
The same logic at texture creation is even written without compressed (line 5735):

    const bool compressed = bimg::isCompressed(bimg::TextureFormat::Enum(m_requestedFormat) );
    const bool convert    = false
        || m_textureFormat != m_requestedFormat
        || swizzle
        ;