Stretched images have incorrect texture coordinates.

[nical]

I haven't figured out why, but it is very easy to reproduce: just add an image with its stretch_size greater than its actual size. This test case is a modified version of the sample from the webrender repo: https://gist.github.com/nical/fee1cfecd8fe0ddd6b9fb644cb360559

It creates an image positioned with a rect of size 500x500, and a stretch_size of size 500x500, but the image itself is 100x100. To make the bug easier to see the image has a 2px border. When running this test case you can see that the right and bottom edges are cutting in the borders.

Interestingly, when adding vec2(5.0, 5.0) to position_in_tile in ps_image.fs.glsl, the image looks like it is rendered properly (it's hard to tell visually if 5 is the precise number that fixes it for this stretched size, but it's definitely suspicious).

[nical]

Actually, it's stretched (more than it should be) rather than offset.
If I fill the inflated borders with another color in texture_cache.rs and accentuate the stretching ratio, the "real" border from the image itself gets cut out in the right and lower sides while the left and upper sides don't show the special color I added around the image in the texture cache.

Also, the uv0 and uv1 sent to the shader look legit.

[nical]

Turns out I was not correctly generating the input images (shame on me!) the bug doesn't actually exist.