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

Reset pixel storage settings after load texture.