QTiffHandler: fix default strip sizes for 64-bit and float images

Commit 8455ab6cb29fcccf44977c517689a8ad26781140 (5.11.3) replaced all
TIFFDefaultStripSize() calls with a new file-static defaultStripSize()
function call, stating

> libtiff's default stripsize is tiny (8KB), so splitting the image
> into such strips on writing would significantly hurt the compression
> rate. Aim for 4MB strips instead.

When commit 2842212e88afb200a0fcfda7d306c4e8eee26407 (5.12) added
support for RGBA64, it was forking off a version of qtiffhandler.cpp
that was pre-8455ab6cb29fcccf44977c517689a8ad26781140¹, so it
continued to use the TIFF default strip size of 8KiB.

When the two were merged together², the two commits didn't conflict
and so RGBA64 continued unchanged. The later float format additions
inherited it by cut'n'paste.

¹ youngest common ancestor: 55904e9671f4e927308a78d217c0069528ba7f49
² oldest common decendant: a9be081dacbd307eb277ef1fe6c095677e00248f

Fix by applying the 4MiB strip size to the RGBA64 and float format,
too.

[ChangeLog][TIFF plugin] Fixed a bug that caused RGBA64 and
float-based formats to be written with 8KiB strips, severely limiting
the compression achievable, compared the 4MiB strip size that the TIFF
plugin uses for all other formats.

Pick-to: 6.12 6.11 6.8
Change-Id: I0af7ae6a0fbccce20e61492a36346548d612a04e
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>