refactoring: adjust Image cache mechanism

[Songmu]

Since Image now has mutable states and fields beyond image data, using it directly as a cache could cause unexpected side effects; therefore, we now cache internalImage instead of the Image itself.

This means we can no longer share caches when different keys have the same data, resulting in a slight reduction in efficiency. However, if necessary, we can continue to optimize this part later.

[github-actions]

Code Metrics Report

	main (4e881b2)	#422 (af44159)	+/-
Coverage	32.3%	32.7%	+0.3%
Code to Test Ratio	1:0.8	1:0.8	+0.0
Test Execution Time	11s	10s	-1s

Details

  |                     | main (4e881b2) | #422 (af44159) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          32.3% |          32.7% | +0.3% |
  |   Files             |             28 |             28 |     0 |
  |   Lines             |           3120 |           3099 |   -21 |
+ |   Covered           |           1010 |           1014 |    +4 |
+ | Code to Test Ratio  |          1:0.8 |          1:0.8 |  +0.0 |
  |   Code              |           6432 |           6403 |   -29 |
+ |   Test              |           5630 |           5695 |   +65 |
+ | Test Execution Time |            11s |            10s |   -1s |

Code coverage of files in pull request scope (36.8% → 37.7%)

Files	Coverage	+/-	Status
apply.go	1.4%	+0.0%	modified
cache.go	100.0%	+30.0%	modified
convert.go	0.0%	0.0%	modified
image.go	67.3%	+6.6%	modified
md/md.go	78.6%	+0.0%	modified
preload.go	0.0%	0.0%	modified

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Reported by octocov