Adding a programmable image shader to gl_hud

- add a framebuffer to gl_hud with programmable image shader
- add optional SSAO image shader to the new framebuffer
- add screenshot to png functionality
- implement in glViewer

note: ptexViewer and some others still need refactoring to use the new hud capabilities
Decrease compiler warning thresholds and fix outstanding warnings (continued)

- turn off some of icc's remarks (mostly because of tbb)
- fix many of icc -w3 remarks (more to fix once i can work around icc 14.0 linker barfing)
Minor fixes to gl_hud and gl_framebuffer

- leave framebuffer off by default (and remove SSAO UI from examples)
- fix some resources leakage when toggling SSAO on/off
Fix framebuffer shader compiling for OSX

The driver apparently does not tolerate #version after a #define
Fix compiler warnings in OsdClVertexBuffer
Fix icc build problems

- icc asserts out because of the length of rpath, so we are temporarily removing the elements
  that aren't defined until chrpath is invoked by cmake (this means installed executables are
  missing paths to the built OSD dsos)
- comment out some of the warnings for missing packages if they were deliberately turned off
Add command line option to CMake's options

It solves the issues when one generated makefiles with
some of the custom command line options passed to CMake
and then modified the CMake file. This used to forget
about settings passed via command line.

Now it's no longer required to re-run CMake with all
custom options every time CMakeLists.txt files are
modified.
Add an option to disable OpenCL

It's not always wanted to compile OpenSubdiv with OpenCL
support when the SDK is installed on the builder machine.
Document NO_MAYA command line option

If was used in CMake but was never documented.
Make PTex support optional

Basically makes configuration log much cleaner on systems
where PTex is not installed.
Don't show warnings when OpenCL component is disabled
Fix compilation error on windows+msvc2013

- Some missing includes of <algorithms> in order to have
  stdd::min() and similar functions.
- Need to cast numIndices and numNVerts to int explicitly
  in order to solve warning treated as an error about
  precision loss.
- Can't do vector[0] for an empty vector, it'll generate
  a runtime range check error.
- MSVC only works fine with make_pair(foo, bar) syntax,
  without explicit template substitution here. Otherwise
  weird 'can't cast int to int&&' errors are happening.
Fixed build warnings/errors with VS2010 Pro
Adding a FindCLEW.cmake module in anticipation of using CLEW as a dependency
Fix real stupid copy-paste typo from a while ago
Merge pull request #304 from Nazg-Gul/dev

Fix real stupid copy-paste typo from a while ago
Integrate CLEW into osd library and examples

If the system has CLEW installed (which is detected by recently
added FindCLEW routines) then OpenSubduv would be compiled against
this library.

It makes binaries and libraries more portable across the systems,
so it's possible to run the same binary on systems with and without
OpenCL SDK installed.

The most annoying part of the change is updating examples to load
OpenCL libraries, but ideally code around controllers and interface
creation is to be de-duplicated anyway.

Based on the pull request #303 from Martijn Berger
Merge pull request #305 from Nazg-Gul/dev

Integrate CLEW into osd library and examples
Reordering includes to address a compile error on mac/glew environment
Fix Hud display for higher DPI (MBP retina)