This commit updates the `nvgBeginFrame()` function to forward the received `devicePixelRatio` parameter to the `renderViewport()` function so it is possible to configure the environemnt correctly when using `bgfx` backend.

This commit implements a new nvgCancelFrame() function for the purpose described in #168.

Currently, if calling nvgBeginFrame(), the nvgEndFrame() function must be called to finish the drawing. However, if knowing the drawing would take too much time and want to give up the drawing. There is no way to cancel it. nvgCancelFrame() gives another choice to end the frame without actually drawing.

- API changed!
- moved image repeat from nvgImagePattern() into image creation
nvgCreateImage*()
- made flip-y and premultiplied common image flags (not just GL)
- removed nvglImageFlags(), flags passed in via
nvglCreateImageFromHandle() flags
- nvgluCreateFramebuffer takes image flags as param

- added function to allow to combine scissor rects using intersection

- added typedefs for all structs
- fixed some compiler warnings

Tweaked premake to remove some windows warnings.

- added nvgQuadTo, draws quadratic bezier curve segment

- 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

- Added alpha for images
- Added global alpha

- fixed basic shape windings
- clamp radius of rounded rect so that it behaves nicely when squashed

- use one not used macro for all

- added glyph min/max x
- fixed first glyph position

- added nvgCurrentTransform() which allows to get the current
transformation matrix
- added skew x/y transforms
- exposed functions 2x3 matrix functions
- exposed point transform
- added deg/rad conversion functions

- nvgTextBounds() API changed, assumes x,y as input too
- fixed nvgTextBounds() return values
- added nvgTextBoxBounds()
- text line width is handle more consistently
- added note about return values of text measure funds
- changed demo to show text(box)bounds usage

- fixed some warnings
- changed nvgLetterSpacing() to nvgTextLetterSpacing()
- added nvgTextBox()
- added nvgTextLineHeight()
- added some documentation

- 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

- changed tessellation flatness test which produces less vertices
(especially when using straight lines)
- added nvgDebugDumpPathCache which allows to dump path cache state to
stout

Currently working with gl3buf backend.

- changed tessellation threshold from 0.3 to 0.25
- fixed scissor AA on retina
- fixed stroke AA on retina

- added support to take screen shots
- added support to render to frame buffer using premultiplied alpha
- fixed spelling NVGaling -> NVGalign

- removed assize from the back-end api
- moved image to paint in renderTriangles call

- added NVG_NOTUSED to suppress unused param warnings
- fixed scissor anti-aliased border width when zoomed

- fixed off-by-one pixels from demo
- added device-pixel-ratio support (retina support)
- added nvgEndFrame()

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

- set swap interval to 0 to better see performance
- draw convex shapes without stencil to speed things up
- improve vertex buffer upload for gl3

- 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

- fixed nvgArcTo
- added nvgTransRGBA, allows to change alpha of color
- added nvgFindFont, allows to find font id by name

- updated font stash
- added end parameter to text (can draw substring)
- text and text measure return advance/width

- fixed arc rendering for small arcs
- added color wheel to example
- added screenshot