vcl: Replace GetPixmapFromScreen by FillPixmapFromScreen

Change-Id: I2cb960d194ee5bc38beece97e8b21cc6fa3b3fbc
vcl: Use backend-independent functions to fill/render pixmap on KDE

Change-Id: I662a05d988f5989fc2ac0d06003851c56105e184
vcl: Implement copyBits in the OpenGL backend

Change-Id: I101afd11394d3d024d38519afd61d700114702ac
vcl: Draw gradient in alpha device even when using native rendering

Change-Id: Iead4ac3e97197f8f863242a2162907b32c3c5b2b
vcl: Improve OpenGLTexture implementation by allowing implicit sharing

Change-Id: I6421265325e72023d1affe671d75488185772786
vcl: Make sure the offscreen texture is unique before rendering to it

Change-Id: I265ce62e983e6f44ba51675993c9ec1f071c160e