vcl: Add dummy Replace to SalBitmap implementations

Change-Id: I2f8b11a3f7cb8872a1d8f6eeeae8ce3f30223496
vcl: Implement bitmap color replacement operation in OpenGL backend

Change-Id: Ia86b67e92985eeb4fb2a5f6cd74c65fab2ac5566
vcl: Acquire framebuffer from current context when reading back texture

Change-Id: I410ac2d10ec2e498d9d8444e5584bfb14727c90b