This commit changes the `nvgluCreateFramebuffer()` function to not assign a view id automatically as it would easily reach the limit of `uint8_t`. Instead, a new `nvgluSetViewFramebuffer()` function is created to allow users to assign the view id manually as mentioned in #1095.
This commit updates bgfx's NanoVG codebase to adapt the latest NanoVG commit since I've implemented blending support for the original NanoVG and there is no need of `nvgState()` anymore.
This commit ports nanovg's helper functions for rendering stuff in framebuffer defined in `nanovg_gl_utils.h`. Also, a new `nvgState()` function is created for bgfx backend so it is possible to blend between framebuffers, which feature is long requested but not yet supported in original nanovg. The `nvgState()` function can be called between `nvgBeginFrame()` and `nvgEndFrame()`.
This commit moves the additional nanovg functions for bgfx to a separate header file, so it is possible to compile the original nanovg source files with bgfx in a custom build environment.
