title: Blending textures in SDL2
body: <p>I have 2 textures, both of them are created with SDL_TEXTUREACCESS_STREAMING flag. They have blend mode set to SDL_BLENDMODE_BLEND. I'm updating both of them in each frame from 2 different surfaces. One of them has mostly alpha values equal to 0 (mostly transparent). When I put one of the textures on the top of another, the blending doesn't happen. </p>
  
  <p>For example - one surface is going to have RGBA values = 255, 255, 255, 0 for all pixels. When I update a texture with it, and put the same texture on the top of another image I get only black pixels.</p>
  
  <p>How do I accomplish blending on the 2 textures in such a way that pixels with 0 alpha will be treated as transparent instead of being turned to black pixels? What is happening? Are Textures ignore alpha values?</p>