Memory leak + small fixes

fix nvgCreateImageFromHandle for GLES2/3.

- 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)

add external APIs to GL backend to get native Texture Id.

Relax stroke width limit

Use highp precision when available for GLES

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

Explicit cast to make C++ compilers happy

- 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

Made FreeType support a bit clearer

Removed GL_BLEND disable and enable inside glnvg__fill() to prevent performance warning.

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

Make sure fons__tmpalloc returns a 16-byte aligned pointer

- 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

Set glFrontFace in glnvg__renderFlush

Removed GLFW 2 option macro

GLFW_NO_GLU is a GLFW 2 feature.  GLFW 3 does not include the GLU header
by default.

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