Started to implement simple pixel transfer operations.

Summary: Added simple implementations of GL functions required by Diana.

Test Plan: Run Diana in non-GL mode and inspect text item styling.

Reviewers: #diana

Differential Revision: https://phab.met.no/D67
Plot the final, processed image rather than the original.
Fixed recolouring of symbols in bdiana.
Attempt to fix colour scaling and biasing for symbol styling support.
Fixed colouring of symbols in bdiana to the extent possible using composition modes.
Ensured that the pixel zoom attribute is defined correctly by default.
Merge branch 'master' of github.com:metno/diana