fix(heatmap): avoid the unexpected gaps when dpr is not an integer

[plainheart]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Fix some gaps will occur in the heatmap when dpr is not an integer.

Fixed issues

• 直角坐标系热力图如何去除网格线 #6011
• 在2K分辨率下，热力图还是会有随机出现的灰色虚线 #12519
• heatmap: there is a tiny gap between each rect item，how to remove it? #11975

Details

Before: What was the problem?

There are unexpected obvious gaps when dpr is not an integer.

After: How is it fixed in this PR?

Add extra 0.5px for each cell.

Comparison

Before	After

Misc

• The API has been changed (apache/echarts-doc#xxx).
• This PR depends on ZRender changes (ecomfe/zrender#xxx).

Related test cases or examples to use the new APIs

• test/heatmap-gap-bug.html
• test/heatmap-large.html

Others

Merging options

• Please squash the commits into a single one when merging.

Other information

[echarts-bot]

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The pull request is marked to be PR: author is committer because you are a committer of this project.

[pissang]

Just for curious, why the previous Math.floor and Math.round didn't fix it well

[plainheart]

why the previous Math.floor and Math.round didn't fix it well

I'm not quite sure but I guess it might be because some decimals are unexpectedly rounded up/down. And except for adding an extra half-pixel, I have no better idea for dealing with it.

[pissang]

The result looks good enough so far.

[echarts-bot]

Congratulations! Your PR has been merged. Thanks for your contribution! 👍