[glsl-out]: combined image sampler (#177)

* Added generation of the texture mappings to the glsl backend

* Added a temporary workaround for the spirv shaders
Fixed some parts that were using vulkan glsl

* Made texture mapping not require a sampler

* Used interface to build the texture map

* Made global names mandatory except for sampler ones

* Addressed more comments

* Removed the temp fix for the spirv frontend

* Addressed the comments