in some situations int and GLsizei is not the same type which leads to conversion warnings

The syntax is wrong when checking defined identifier.

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.

Conflicts:
	src/nanovg_gl.h

- 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 NVG_DEBUG which can be used to omit calls to glGetError()
- removed return value for glnvg__checkError
- create flag defines to enum

These changes make it possible to integrate nanoVG into other openGL libraries like Cinder and openFrameworks and be able to draw afterwards.

- added typedefs for all structs
- fixed some compiler warnings

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

- added (optional) 3-pass stencil trick to stroke rendering, reduces
overdraw artifacts, a little slower
- fixed some text bounds issues

- Added alpha for images
- Added global alpha

- fixed flags setup in nvglCreateImageFromHandle
- changed frame buffer to only compile on GL3
- changed nvgluCreateFramebuffer() to return FBO struct instead of
filling existing one (for consistency)

- use regular uniforms for once-per-frame values
- set tex sampler when using NANOVG_GL_USE_UNIFORMBUFFER

"Fragment Shader is going to be recompiled because the shader key based on GL state mismatches."

- changed remainder of allocations to follow same 1.5x patters
- do not leak memory when realloc() fails
- added error handling in case allocations fail

- increased initial buffer sizes
- change all allocator patterns to over allocate by 1.5x
- fixes bug where nvgl back-end was not allocating +n as requested

- make default uniform struct align 4

- 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

- added documentation about changed GL state
- made old GL2 and GL3 backends obsolete

- made uniform buffer support behind separate define for easier toggling
- calculate scissor and stroke alpha outside type selection to simplify
the shader (or else it produces rendering artefacts)
- restore default pixel store settings instead of restoring old state
- set active texture to 0 when starting to render a frame

- added support for GL(ES)2 for nanovg_gl3buf (something is still not
quite there)
- nanovg now sets all required GL state
- removed compiler warnings about unused vars

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