Combine most common shaders for non-texture drawing and texture
drawing into two combined shaders. Inside the shader we switch
between the code paths with if statements.

Using if statements (or any other branching statements) is
discouraged inside shaders but on the other hand we reduce program
state changes if we have less shader changes - which is more
important for us as we want to push more work to the GPU.

Change-Id: I6701b93faa9b0f55dd0af6d983ce4c2de4539c70
Reviewed-on: https://gerrit.libreoffice.org/25357Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>