Fix legend icon not rendering correctly due to wrong scaled dpr metric

[Adrian-B-Moreira]

Fix QwtNullPaintDevice.metric() returning the wrong value for PdmDevicePixelRatioScaled, which caused legend icons to collapse to a degenerate size.

This is similar to #104, but with different causes.

Qt stores the device pixel ratio as a fixed-point integer scaled by QPaintDevice::devicePixelRatioFScale() (which is 0x10000 = 65536). This is how Qt represents a fractional DPR (e.g. 1.25, 1.5, 2.0) while keeping metric()'s return type as int.

So returning 1 for PdmDevicePixelRatioScaled means DPR interpreted as 1/65536 ≈ 0.0000153 → collapsed.

This also prevents the "Device has no metric information" warning.

Before the fix:

After the fix:

[PierreRaybaut]

Thanks a lot @Adrian-B-Moreira for the feedback and for the fix!

[PierreRaybaut]

Hey @Adrian-B-Moreira, we already have a few fixes since v0.16.2. If you want, we can release it quickly. Unless you may have other things to fix or report? Thanks.