Add fill paint support to nvgText.

[starwing]

Hi Mikko :)

Currently nvgText can only draw colored text, but can not draw image pattern, or gradient. So I want to propose that add support to use whole paint when draw text.

the plan is that: add a int textureImage parameter to renderTriangles, and add a extra texture unit in Uniform buffer, first get color from font texture, then calculate the paint color, and do fix two color to produce the final color.

I have not good at texture unit on Unifom buffer, So can you made that? Or give me some advise and I will try to make a pull request.

regards

[EriKWDev]

Would be very neat indeed xP