public void Begin(Matrix projectionMatrix, Matrix transformMatrix)
{
if (_inBegin)
throw new InvalidOperationException("End must be called before Begin can be called again.");
// Defaults
_blendState = BlendState.AlphaBlend;
_samplerState = SamplerState.LinearClamp;
_depthStencilState = DepthStencilState.None;
_rasterizerState = RasterizerState.CullNone;
_projectionMatrix = projectionMatrix;
_transformMatrix = transformMatrix;
SetupMatrix();
_inBegin = true;
}