… texture function. r=cbrewster

The `sampleInUvRect` function in `gfx/wr/webrender/res/cs_svg_filter.glsl` calls
`texture`, passing the optional third argument that specifies a bias to the
sampler's level-of-detail calculation, used for mipmap interpolation. However,
the `texture` override in `gfx/wr/swgl/src/glsl.h` that accepts this parameter
is stubbed out, with an `assert(0)`. This causes twelve tests in
`gfx/wr/wrench/reftests/filters` to crash.

Nothing in Firefox uses mipmaps, and Servo doesn't use Software WebRender, so it
should be fine for that call to `texture` to simply not pass the level-of-detail
bias.

This patch makes that change.

Differential Revision: https://phabricator.services.mozilla.com/D69867

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/89e164da5af7b9a9152deacdb06bb5b36d3bf2b8