Add support to emrun to receive binary file dumps, and add a JS function emrun_file_dump() when building with --emrun that performs a data dump back to server. Used e.g. to debug issue #1826.
Fix the reference image sdl_canvas_alpha.png. The previous image was an example of broken rendering: the test does (additive) alpha blending, and in the previous image some of the pixel components wrapped around to show green color artifacts. Fixes #1826.
Merge branch 'fix_test_sdl_canvas_alpha' of https://github.com/juj/emscripten into incoming
increase slack on test_sdl_canvas_alpha