Process Background Image merge tag

[jestonihpi]

Description

This PR should solve the issue #1406.

• I have added $form and $entry to View_PDF's load_core_template_styles method. This will enable us to use process_tags.
• Added an if statement that checks if the background-image value contains an merge tag before proceeding.

#1406

Testing instructions

• Setup a Core PDF on the form
• In the Background Image setting insert a URL to an image in the media library
• Add a merge tag to the image URL
• View the PDF and append URL with ?html=1
• View the page source and see the merge tag was not correctly converted to the associated value

Screenshots

Checklist:

• [x] I've tested the code.
• [x] My code is easy to read, follow, and understand
• [x] My code follows the accessibility standards.
• [ ] My code has proper inline documentation / docblocks.

Additional Comments

[codecov[bot]]

Codecov Report

Merging #1407 (dee8bfa) into development (55049ad) will decrease coverage by 0.95%. The diff coverage is 100.00%.

:exclamation: Current head dee8bfa differs from pull request most recent head 9d5879f. Consider uploading reports for the commit 9d5879f to get more accurate results

@@                Coverage Diff                @@
##             development    #1407      +/-   ##
=================================================
- Coverage          78.29%   77.34%   -0.96%     
+ Complexity          2901     2900       -1     
=================================================
  Files                243      243              
  Lines              10118    10108      -10     
  Branches             370      370              
=================================================
- Hits                7922     7818     -104     
- Misses              2188     2282      +94     
  Partials               8        8              

Impacted Files	Coverage Δ
src/View/View_PDF.php	79.87% <100.00%> (+0.12%)	:arrow_up:
src/View/html/PDF/core_template_styles.php	98.00% <100.00%> (ø)
src/Helper/Fields/Field_Form.php	0.00% <0.00%> (-89.37%)	:arrow_down:
src/Helper/Fields/Field_Option.php	41.86% <0.00%> (-58.14%)	:arrow_down:
src/Helper/Fields/Field_Product.php	64.00% <0.00%> (-36.00%)	:arrow_down:
src/Helper/Fields/Field_Total.php	60.86% <0.00%> (-30.44%)	:arrow_down:
src/Helper/Fields/Field_Quantity.php	71.42% <0.00%> (-28.58%)	:arrow_down:
src/Helper/Fields/Field_Shipping.php	65.38% <0.00%> (-26.93%)	:arrow_down:
src/Helper/Helper_Field_Container_Gf25.php	84.00% <0.00%> (-6.00%)	:arrow_down:
src/Helper/Fields/Field_Quiz.php	88.88% <0.00%> (-2.78%)	:arrow_down:
... and 2 more

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[jestonihpi]

Hi @jakejackson1 , this pr is now ready for review. Thank you.

[jestonihpi]

Thanks for the feedback @jakejackson1, added this on my today's list.

[jestonihpi]

I have applied the requested changes @jakejackson1 , kindly review this pr please.