Add support for binding to a multisampled framebuffer

[brackeen]

This allows binding to a multisampled framebuffer, useful for drawing 3D content with MetalNanoVG.

Example:

id<MTLTexture> renderTargetTexture; // Texture where sampleCount > 1
NVGcontext *nvg;
...
MNVGframebuffer framebuffer;
framebuffer.ctx = nvg;
framebuffer.image = mnvgCreateImageFromHandle(nvg, (__bridge void *)renderTargetTexture, 0);
mnvgBindFramebuffer(&framebuffer);