SkPDF: Join Positioned Text

When N sequential positioned glyphs differ in positions by exactly the
advances of the first (N-1) glyphs, join the glyphs into a string
rather than changing the text matrix between each glyph draw.

Decreases PDF output size by about ~1.4%.  Potentially more on
text-heavy pages.

A single-typeface PDF of an 27kB ASCII document shaped with harfbuzz:
        before:  187743 Bytes
         after:  65513 Bytes
    difference:  -65.1%

Before:
    BT
    /F0 13 Tf
    1 0 0 -1 143.5 61 Tm
    <0029> Tj
    1 0 0 -1 150.634765 61 Tm
    <004C> Tj
    1 0 0 -1 154.602050 61 Tm
    <0055> Tj
    1 0 0 -1 160.245117 61 Tm
    <0048> Tj
    1 0 0 -1 167.925781 61 Tm
    <004B> Tj
    1 0 0 -1 176.469726 61 Tm
    <0052> Tj
    1 0 0 -1 184.518554 61 Tm
    <0056> Tj
    1 0 0 -1 190.980468 61 Tm
    <0048> Tj
    ET

After:
    BT
    /F0 13 Tf
    1 0 0 -1 0 0 Tm
    143.5 -61 Td <0029004C0055> Tj
    16.7451171 0 Td <0048004B005200560048> Tj
    ET

Also: update the Text matrix with the `Td` operator, instead of
overwriting it with the the `Tm` operator.  In the worst case, when
every glyph is positioned differently than it's advance, this still
makes the command stream smaller:

Before:
    ...
    1 0 0 -1 58.328125 660 Tm <0055> Tj
    1 0 0 -1 61.609375 660 Tm <004C> Tj
    1 0 0 -1 63.828125 660 Tm <0056> Tj
    ...

After:
    ...
    3.140625 0 Td <0055> Tj
    3.28125 0 Td <004C> Tj
    2.21875 0 Td <0056> Tj
    ...

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2150393002

Review-Url: https://codereview.chromium.org/2150393002

0 files changed, 0 insertions, 0 deletions