New uniform update mode for GLES2 NANOVG_GL_USE_UNIFORMARRAY

This adds a new uniform update mode for GLES2 (non-uniform-buffers) which replaces the 13 fragment shader uniform updates with a single glUniform4fv() call.

I tested all possible modes as good as I can on OSX and WebGL, but the should also be tested thoroughly on GL2 and GLES3.

I also kept the old code in as default for GL2 (and GLES3 as far as I can see), it probably would make sense to have only 2 uniform update modes, uniform buffer or uniform array, and ditch the old mode.