We already know how to produce pork with skin that bright during the night. The most advanced project Enviropig attempted to enhance phosphorus absorption.
The aim would be to make the skin produce glucose with the carbon dioxide in the air (along modifying other organs for hormonal regulation).
Is it possible with technology of the next decades ?
If not, why ? If yes, how much this would allow to reduce food requirements for feeding them ? (the pig farming would be performed under greenhouse)
According to Wikipedia's article on photosynthetic efficiency, sugarcane, which is abnormally efficient, can store 8% of the energy of sunlight it receives. Let's say that your genetic engineering is pretty good, so the pigs are able to reach this level of efficiency.
Sunlight is 120 W/m2, which is equivalent to 0.029 Calories per second, or about 104 Calories per hour of sunlight. Let's say there are 10 hours worth of sunlight during the day, both for ease of calculation and to account for dawn and dusk not having as much light. That leaves us with a potential of about 1000 Calories per day of sunlight.
Given that pigs can grow to almost 2 meters long, and that they're longer than they are wide, let's just say the pigs will have 1 m2 of exposed skin.
Putting these together, your pig will get about 80 Calories per day from sunlight. I found this source that says an adult pig should get 9000 Calories per day (I think this is for pigs that are being raised for meat). That's less than 1% of their daily caloric needs.
Based on these numbers, I don't expect this to happen any time in the near future. It would take a lot of time and effort in order to figure out how to get the 8% efficiency of sugarcane instead of the more typical 1% efficiency of most crops. Then, once you've reached that point, you get a very marginal benefit.
In theory, eventually, anything is possible with genetic engineering. Someday perhaps we will be able to mix and match anything that is biologically possible into an organism. But this technology is still in its infancy and it has a long way to go.
The two examples you give are very simple modifications. The glowing skin comes from the expression of a single gene called GFP, and the Enviropig attempt also appears to be the introduction of a single gene from an E. coli species. These efforts are becoming cheaper and easier with new technologies, but are still multi-year, million-dollar projects for the most part. Your project would require the transplantation of an entire organelle, the chloroplast where photosynthesis takes place, into an organism that it has no business being compatible with. The chloroplast even has its own genome adding to the complexity.
That said, the main issue with larger genetic engineering projects like yours is not really the modification of the genome itself. Already we can change just about any sequence in a genome to any other sequence using techniques like CRISPR and that will likely get cheaper and easier. No, the main barrier to huge genetic modifications is that we have no idea how to actually make it work. We just don't know nearly enough about how the tens of thousands of different genes, proteins and other molecules interact to form a functioning chloroplast to ever actually get it to work. That's even without considering how all of those elements will interact with the host organism's physiology. Science advances our understanding of these things bit by bit, often by performing simple genetic modifications, adding a gene or deleting a gene, to see what happens. This process is quite slow and is a long way off from generating the understanding necessary for your project.
So, near future? Almost certainly not. Distant future? Sure, but then hopefully we can also do things that are a lot more interesting that photosynthetic pigs.
