Reduced overhead of calling memoryUsage() on SimpleTypedData.

The new implementation is approximately 2x faster than the previous one.
Merge pull request #343 from johnhaddon/memoryUsageOptimisation

Reduced overhead of calling memoryUsage() on SimpleTypedData.
Removing op wrapper for dead op
Bumped version to 9.0.0-a7 and updated Changes file
Merged: Fixed precision issue with computing the projection matrix for dsm depth remapping, which was causing deep holdout problems.

Conflicts:
	src/IECoreRI/SHWDeepImageReader.cpp
Merged: Applied same fix to shw writer as reader.
Hacked up constants in test case to pass again.

Conflicts:
	src/IECoreRI/SHWDeepImageWriter.cpp
turning ri:automaticInstancing on by default
Merge pull request #347 from danieldresser/shwPerpFix9

Applying shw conversion precision fixes to master
stopped IECoreMaya::LiveScene::readObject() returning null pointers
Merge pull request #349 from davidsminor/mayaSceneNullObj

stopped IECoreMaya::LiveScene::readObject() returning null pointers
stopped IECoreMaya::LiveScene::readAttribute() returning null pointers
Merge pull request #350 from davidsminor/mayaSceneNullAttrs

stopped IECoreMaya::LiveScene::readAttribute() returning null pointers
Fixed bug in IECoreRI::Renderer camera editing.

It was incorrectly storing m_lastCamera (the primary camera) even during edits, meaning that if cameras other than the primary camera were edited, it would lose track of the primary camera. For common sequences of edits (all cameras in order, primary last), this meant the main world camera would never be updated.
Merge pull request #351 from johnhaddon/multipleCameraEditsFix

Fixed bug in IECoreRI::Renderer camera editing.
Bumped alpha version and updated Changes.
TIFFImageReader now checks if the tif is tagged as coming from tdlmake, and if so, sets the sourceColorSpace from the image description set by tdlmake.
Switched glsl diffuse and specular functions to match 3delight's properly normalized BRDFS

I'm open to putting these under a new name if we want to keep the old versions - if we overhaul how lights are passed, we'll be changing some of this anyway ( like the use of gl_MaxLights ).  But for the moment, this is a fairly nice high quality spec BRDF.  Maybe a little bit too high quality - the Smith G approximation I'm using might be a bit overkill for the viewport, but it's not that complex, and we need something to kill off spec on the back.
fixed crash in SceneShape::hasSceneShapeObject for invalid scene
Merge pull request #353 from davidsminor/mayaHasSceneShapeObjectCrash

fixed crash in SceneShape::hasSceneShapeObject for invalid scene
Merge pull request #348 from davidsminor/autoInstancingDefaultOn

turning ri:automaticInstancing on by default