Tweaked premake to remove some windows warnings.

- fixed stroke cap tessellation amount calculation

- 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 (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 glyph position bounds for whitespace

- 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)

- bezier tessellation was using second control point instead of last
point in error calculation

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

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

- fixed copy/paste bug in bevel calculations

- line caps did not consider AA width

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

- use one not used macro for all

- 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

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

- scale and letterspacing were mixed in call to getQuad

- 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

- added test for multiple stroke widths
- allow stroke width down to 1px size, then scale with alpha
- fixed textBoxBounds() in hi-dpi