Added nvgTextBoxBounds, fixed nvgTextBounds

- 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
Added TODO about GLES2 pixel shader precision
Allow small strokes, fixed textBoxBounds()

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

- added documentation about changed GL state
- made old GL2 and GL3 backends obsolete
Update README.md
Fix for issue #14
Added nvgCurrentTransform() and exposed matrix functions

- 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
fixes texture formats for GLES2/GLES3
Fix for issue #69

- make default uniform struct align 4
Merge pull request #70 from andrewcorrigan/master

fixes texture formats for GLES2/GLES3