Remove image borders

[kvark]

Fixes #789

Clamping the texture coordinates in pixel shaders (as well as using textureLod) to ensure sampling within original borders now.

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This change is 

[kvark]

r? @glennw

BTW, I removed the distinction between requested_size and allocated_size now, following the general idea of relying on git history to restore the obsolete stuff if needed.

[glennw]

This generally looks good to me (I'll run the CSS tests locally to confirm). We'll need to apply the same clamp in the text run shader though.

[kvark]

Good point @glennw , the text is now also converted.

[bors-servo]

☔ The latest upstream changes (presumably #776) made this pull request unmergeable. Please resolve the merge conflicts.

[glennw]

This looks good, but will need to be rebased after the external texture images changes landed.

[kvark]

@glennw rebased now

[glennw]

Nice one!

[glennw]

@bors-servo r+

[bors-servo]

📌 Commit 5b41357 has been approved by glennw

[bors-servo]

⌛ Testing commit 5b41357 with merge 2f17a52...

[bors-servo]

☀️ Test successful - status-travis
Approved by: glennw
Pushing 2f17a52 to master...

[JerryShih]

@kvark @nical
Are we use mipmap now? I don't see any mipmap generating in WR. So, I'm curious about the textureLod usage here.

[kvark]

@JerryShih there are 2 reasons for using textureLod in this case:

1. It clearly states the pre-condition. Since the image border code uses a half-texel offset, it's not going to work properly for any Lod > 0.0. So now if you add any mipmap levels, the code doesn't care and will still work properly.
2. It may be slightly faster. The shader compiler doesn't know if a texture has mipmap levels or not (unless it re-compiles the shader on the fly), so it will generate the gradient instructions and a regular sample, only to be dismissed by the texture sampler that discovers no mipmap levels. Using textureLod directly produces the efficient HW shader assembly right away.

[leeoniya]

@kvark maybe worth adding these architectural decisions to the wiki? probably makes sense for any details that can answer "why not use X?" for common graphics tech.

[kvark]

@leeoniya that should be a good start:
https://github.com/servo/webrender/wiki/Graphics-Features

[leeoniya]

excellent, thanks 👍