gtk_shader_builder_add_define should check both define_name and
define_value for not-NULL and not-empty, but the second precondition
check checks define_name again for not-empty-ness.
We want the GL driver to cache as many resources as possible, so we can
always ensure that we're in a consistent state, and we can handle state
transitions more appropriately.
We should use ShaderBuilder to create and store programs for the GL
renderer. This allows us to simplify the creation of programs (by moving
the compilation phase into the ShaderBuilder::create_program() method),
and move towards the ability to create multiple programs and just keep a
reference to the program id.
GskShaderBuilder is an ancillary, private type that deals with the
internals of taking GLSL shaders from resources and building them,
with the additional feature of being able to compose shaders from a
common preamble, as well as adding conditional defines (useful for
enabling debugging code in the shaders themselves).
