[google_maps_flutter] Fix markers NPOT sampling issue for IOS

[cuteUtya]

This PR fixes bug explained in this issue

• Fixes incorrect or swapped custom marker icons on iOS when decoded bitmaps or target sizes
  are not power-of-two

Expected behavior:

Actual behavior (before PR):

Pre-Review Checklist

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[gemini-code-assist]

Code Review

This pull request updates the iOS Google Maps plugin to address issues with custom marker icons by padding them to the next power-of-two size. The changes include a new utility function for power-of-two calculations and updated image scaling logic in FGMImageUtils.m, along with corresponding test updates. Review feedback identifies a potential orientation bug and notes that padding the image will cause marker anchor displacement. It was also suggested to return the original image directly when it is already a power-of-two size to avoid unnecessary object creation.

[gemini-code-assist]

There are two significant issues with this return statement:

1. Orientation Bug: The potCGImage was created by drawing the original image into a context via drawInRect:, which already handles the image's orientation. The resulting bitmap is already in the 'Up' orientation. Re-applying image.imageOrientation here will cause the orientation to be applied a second time, leading to incorrectly rotated markers if the source image had orientation metadata (e.g., from EXIF).
2. Anchor Displacement: Padding the image to a power-of-two size changes the logical dimensions of the UIImage. Since marker anchors in the Google Maps SDK are normalized (0.0 to 1.0) relative to the image size, this change will cause markers to be misaligned on the map. For example, a 20x20 icon padded to 32x32 with a bottom-center anchor (0.5, 1.0) will now be anchored at (16, 32) instead of (10, 20) relative to the artwork. To fix this, the plugin would need to adjust the marker's groundAnchor to compensate for the padding.

Suggested change

	return [UIImage imageWithCGImage:potCGImage scale:image.scale orientation:image.imageOrientation];
	return [UIImage imageWithCGImage:potCGImage scale:image.scale orientation:UIImageOrientationUp];

[gemini-code-assist]

If the image is already at the target power-of-two size, you can return the original image directly instead of creating a new UIImage instance from the same CGImage.

  if (alreadyPOT) {
    return image;
  }

[cuteUtya]

Apologies in advance if I made some guideline mistakes; I rarely contribute to open-source. However, google_maps_flutter is vital to my work, and I've encountered other similar issues with markers functionality in the past (that was resolved by contributors is past years)