To get nanovg to link correctly under OS X 12.3, it needs to be linked against
the Carbon libraries.  This was missing in the premake4.lua files.  Now fixed.

Tweaked premake to remove some windows warnings.

- all colors in the shader is handled as premultiplied
- removed alphaBlend param from nvgBeginFrame(), it became obsolete
- added NVGL_TEXTURE_PREMULTIPLIED flag
- separated FBO example
- FBOs are treated as premultipied for correct alpha
- changed FBO storage from depth/stencil 24/8 to stencil 8
- moved discard to end of shader which restores the render speed again

The GLFW library path and link flags may differ between Linux
distributions.  This makes the build system use the GLFW pkgconfig file,
which hides such differences.

- added debug configs for the lib
- added more data for font stash text iterator (ptr to current and next
position in string, code point)
- renamed nvgVertMetrics() to nvgTextMetrics()
- added nvgTextGlyphPositions() which returns glyph x position for the
text
- added nvgTextBreakLines breaks a text into multiple rows based on
newlines and max row width
- tweaked tessellation tolerance

- moved performance counter and drawing to separate file
- tweaks to performance drawing

- added option to turn off AA fringe generation
- added examples which use no AA fringes and  MSAA

- link to framework CoreVideo on OSX, fixes compiling on OSX 10.9

- moved demo stuff to separate file
- created two separate examples for gl2 and gl3
- initial stab at OGL 3.2 core profile back-end (currently twice as
slows as 2.0)
- API change: glBeginFrame() takes view witdth and height as input
- added FPS counter to demo