Add love.graphics.getQuadIndexBuffer.

love's pre-generated quad index buffer is useful when combined with drawShaderVertices. Note that it uses 16 bit indices so it can only draw up to 16k quads in a single call.

src/modules/graphics/Buffer.cpp

@@ -36,6 +36,7 @@ Buffer::Buffer(Graphics *gfx, const Settings &settings, const std::vector<DataDe
 	, usageFlags(settings.usageFlags)
 	, dataUsage(settings.dataUsage)
 	, mapped(false)
+	, immutable(false)
 {
 	if (size == 0 && arraylength == 0)
 		throw love::Exception("Size or array length must be specified.");