A few minor fixes:
- In FramebufferRenderbuffer, if renderbuffer isn't 0,
  it must be a valid object.
- In FramebufferTexture2D, I moved the isCompressed check AFTER
  the textarget validation, to avoid using an invalid textarget.
- In GetFramebufferAttachmentParameteriv, not using GL_BACK,
  GL_DEPTH or GL_STENCIL for the default framebuffer should
  produce GL_INVALID_ENUM instead of GL_INVALID_OPERATION.
- In GetFramebufferAttachmentParameteriv, when
  attachmentObjectType is GL_NONE, in ES3, the query for
  GL_FRAMEBUFFER_ATTACHMENT_OBJECT_NAME should return 0.

Change-Id: I913cadd5961fa473b54ddfe174772bb7270dfdc5
Reviewed-on: https://swiftshader-review.googlesource.com/4333Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>