textor | Experimental Text Editor using HTML5 canvas | Canvas library

I am trying to render some polygons to a texture, and then render the texture to the screen. I'm not sure how to debug my code since that would require to probe the internal state of OpenGL, so I would appreciate tips on how to debug myself more than pointing out the error I have done. Anyway, I commented the code I wrote explaining what I expect each line to do.

Here is a description of what the code is supposed to do. Basically, I made a vertex shader that provides the position, UV and color to the fragment shader. The fragment shader has a uniform to activate texture sampling, otherwise it will just output the input color. In both cases, the color is multiplied by a uniform color. First I create a texture, and I fill it with red and green raw pixel data to test. This texture is correcly rendered to the screen (I see the red and green part correctly as I initialized it). Then i try to do the actual rendering on the texture. I try to render a small blue square in the middle of it (sampler disabled on the fragment shader, color uniform set to blue) but I can't get this blue square to appear on the rendered texture.