Better test coverage for SolidBrush + blenders

Performance improvements in the PNG filtering (roughly 20% gain on my machine).

Some minor performance improvements after identifying bottlenecks in VS profiler.

Moved the division out of the loop, division is expensive.

- precalculate anything that's independent of the pixel coordinate

Improve CQ, Round 2

Update IPath.cs

- move to GradientBrushes namespace
- add abstract base class for Gradient Brushes, that impleme...

The current implementation lacks accurrancy and prefers to be fast.
For usual color gradients th...

somehow these don't look correct yet, containing hard edges sometimes.

the theory lacks color checks yet that should be added, but it produces output images for visual...

test images don't have to be that big for axial gradients.
It's sufficient to show they're const...

Issue-532: Added IPixel.PackFromArgb32 and ToArgb32.

as proposed by @antonfirsov: improving readability and memory locality.

thanks @tocsoft for investigation and finding the bug.
He proposed to just replace < by <= in li...

not yet working - what's wrong?

Improve Gif CQ, Part 2

Also removes the duplicate PackageReference

note: v15.7 continues to work on older VS versions

Improve PDFJs Jpeg Decoder

Update System.Memory to preview2

Readme.md: early documentation reference

Extend AppVeyor coverage on more frameworks and platforms

update external dependencies + fix RectangularPolygon usages

update SixLabors.Core reference to 1.0.0-beta0005

Benchmark both Core and CLR

README.md: fix link [skip ci]

Refer to Contribution Guide from README.md

Update logo and readme

Fix spelling.

Rename Blend to DrawImage