The framebuffer_object spec isn't clear in defining whether attaching a
texture as a renderbuffer with mipmap filtering enabled while the mipmaps
have not been uploaded should result in an incomplete framebuffer object.
(different drivers make different decisions)
To avoid an error with drivers that do consider this a problem we explicitly
set non mipmapped filters before calling glCheckFramebufferStatusEXT. The
filters will later be reset when the texture is actually used for rendering
according to the filters set on the corresponding CoglMaterial.
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