Replaced hardcoded GLFW link flags with pkgconfig

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.

- scale and letterspacing were mixed in call to getQuad

add support for MinGW.

fixes texture formats for GLES2/GLES3

- 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

- 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

- 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

- calculate all text related stuff using scaling
- removed a couple of compiler warnings
- tweaked caret picking

- 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

Merged NVGcolor, which implements the color as floats, colors can be > 1.0, allowing HDR rendering when needed.

Currently working with gl3buf backend.

Variable initialization, unused var removal, and casts.

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

Removed some unused variables to silence compiler warnings, initialized
variables to 0, added varialbe casts to silence additional compiler
warnings.

- merged font stash optional FreeType support

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

fixed error return value

- added better logic for inner bevels
- tweaks to reduce overdraw at edges
- fixed convex shape logic

reduces warnings