Add test for three color types of PNG with tRNS chunk

[mloskot]

The tests cover issue #117 and pull request #118, extending the coverage
for all color types that may have tRNS chunks:

• color type 0 (grayscale), the tRNS chunk contains a single gray level
• color type 2 (truecolor), the tRNS chunk contains a single RGB color
• color type 3 (indexed color), the tRNS chunk contains a series of
  one-byte alpha values

Add sample PNG files from the "official" test-suite for PNG created by Willem van Schaik.

Add <define>BOOST_GIL_IO_TEST_ALLOW_READING_IMAGES to Jamfile for PNG tests.
NOTE: Deliberately not defined BOOST_GIL_IO_TEST_ALLOW_WRITING_IMAGES as it seems to require some extra configuration, currently it causes some tests to fail. So, Jamfile likely needs more updates and clean-up.

References

• Color types w/ tRNS discussed in Add test for three color types of PNG with tRNS chunk #125 (comment)
• PNG suite files with tRNS chunks identified using png-chunk-extraction.go program.
• Closes How to add tests for PNG grayalpha fix in #118 #120

Tasklist

• Find a good PNG test image with tRNS chunk
• Complete the test to verify tRNS has been read and used as expected
• Review
• All CI builds and checks have passed

[chhenning]

Is the image read correctly?

[mloskot]

@chhenning It is read successfully, without any exception about incompatibility. It also causes reading of the tRNS chunks, then setting png_set_tRNS_to_alpha.

That is all I observe. I haven't tried to verify if the image is read correctly.

[mloskot]

NOTE: None of the CI builds run the full IO tests, but the simple, hence this added read_alpha_palette_with_trns_chunk test is not checked on our CI-s.

[mloskot]

@stefanseefeld & @chhenning any feedback on this one?

[mloskot]

In order to properly test tRNS support, we may need three different images:

From http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html:

The tRNS chunk specifies that the image uses simple transparency: either alpha values associated with palette entries (for indexed-color images) or a single transparent color (for grayscale and truecolor images).

For color type 3 (indexed color), the tRNS chunk contains a series of one-byte alpha values, corresponding to entries in the PLTE chunk:

Alpha for palette index 0:  1 byte
Alpha for palette index 1:  1 byte
...etc...

For color type 0 (grayscale), the tRNS chunk contains a single gray level value, stored in the format:

Gray:  2 bytes, range 0 .. (2^bitdepth)-1

For color type 2 (truecolor), the tRNS chunk contains a single RGB color value, stored in the format:

Red:   2 bytes, range 0 .. (2^bitdepth)-1
Green: 2 bytes, range 0 .. (2^bitdepth)-1
Blue:  2 bytes, range 0 .. (2^bitdepth)-1

[mloskot]

Following second round of research, #125 (comment) around a month ago, the test received collection of PNG files with the variety of tRNS.

Despite long time, no reviews have arrived, so I'm going to merge it.