GPUMaterial: Group all colorband texture together

This lower the use of texture samplers slots and let users use more real
textures in their shaders.

This patch also make the ramp texture 16 bit floating point. Meaning you
can now use value greater than one in your color ramps.

With the limit of 128 colorband per shader (a color band being either a
color ramp, a wavelength node or a curve node (and maybe wavelength node in
the future)).

Only drawback with the current implementation is that it does not remove
colorband from pruned GPUNodes but it shouldn't really matter in practice.

This should fix T56010

source/blender/gpu/intern/gpu_codegen.h

@@ -197,6 +197,7 @@ void gpu_codegen_exit(void);
 
 const char *GPU_builtin_name(GPUBuiltin builtin);
 void gpu_material_add_node(struct GPUMaterial *material, struct GPUNode *node);
+struct GPUTexture **gpu_material_ramp_texture_row_set(GPUMaterial *mat, int size, float *pixels, float *row);
 int GPU_link_changed(struct GPUNodeLink *link);
 
 #endif