wined3d: Issue an error when the render target is read back without SFLAG_INDRAWABLE being set.

[Felix Nawothnig]

This would have saved me quite some time tracing down the bug I fixed in 
the last patch. Hope the semi-abuse of lastActiveRenderTarget is okay 
with the comment.
---
  dlls/wined3d/surface.c |    9 ++++++++-
  1 files changed, 8 insertions(+), 1 deletions(-)
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