Add uint8_t -> float SRGB conversion function (using LUT)
Add drawcall flags for clearcolour/cleardepth
Add externally visible API to query for pixel history
First pass of pixel history impl (not complete)

* Uses occlusion queries to determine which events touch a given pixel
  (by replaying with all tests disabled except scissor, and scissoring to
  the pixel).
Create a texture to store before/after values as we go along

* In the case where blending is disabled this gives us the passing shader
  output value (when blending is enabled we need to get the in between
  fragments, if any.
Check whether tests are on/off and detect trivial test failures
Interpret/convert pixel values as appropriate to fully expanded form
For each matching event determine which tests it fails (if any).
Add D3D11 driver side call to fetch drawcall by event ID
Allow specifying a background colour per-column on a TreeListNode
Further progress on pixel history view

* Added simple UI
* Don't apply SRGB curve to alpha data
* Account for clear calls not returning occlusion query results, and for
  depth/colour clears.
Test shader discard first (unconditionally) and drop dirty flags

* Also do a full replay after the shader discard test as there seems to be
  some kind of D3D/driver bug that can cause false positives on test
  failures.
Change priority of node background colours, so select colour works