Improved texture allocation

Mark a texture as 'stolen' when it has been stolen by another filter.
Do not allow 'stolen' textures to be reused.
Only reuse textures if "f.LastDependentIndex < FilterIndex" to prevent a shader from reading and writing to the same texture.
Added a dummy filter to SourceFilter to allow OutputRenderTarget to be reused, 

This method should probably be replaced by something nicer, but it seems to work (more or less). For example, using 8 shaders will cause textures to be reused in the following way:
filter 2 steals texture of filter 0 
filter 3 steals texture of filter 1 
filter 4 steals texture of filter 2 
filter 5 steals texture of filter 3 
filter 6 steals texture of filter 4 
filter 7 steals texture of filter 5 
(Note this is what actually happened, not just theoretical)

This way I also discovered another bug: AllocateTextures is called before SourceFilter.OutputTexture has a value, making it impossible to reuse that texture (note that texture -1 is never 'stolen'). Maybe call AllocateTextures just before rendering? (allocating textures seems to be fast enough for this to work)
Same thing happens to the OuputDummy filter (texture -2).
Merge pull request #4 from Shiandow/patch-2

Improved texture allocation