vcl: Don't create new contexts for Virtual Devices on Windows

Change-Id: I561a8142f986aca89e796ce2c4a0902fae41f9e6
vcl: Release the OpenGL context for offscreen rendering after each operation

Change-Id: If253a4c0a1f64b1cc54e0079d4455abf39620ac0
vcl: Add reference on default window's context so it doesn't get destroyed

Change-Id: Ifce8903ce75b1d2ff6acfd717c689f8893d80802
Add dummy Erase to SalBitmap implementations

Change-Id: I5790deb9ab5af6f8678c964af5321264d3bce733
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
vcl: Fix color-replacement fragment shader

Change-Id: I82a0a45961ea3f0ceca7dab67c8736b5e1205bb0