Fix "Mix RGB" mode color leaking into single "Backlight" mode color

[P4ntomime]

Description

Mixed RGB mode sets rgb_matrix_config during rendering but never reverts it to its original state from before / other effects. Proposed in this PR is that both hsv and speed values are saved before rendering mixed rgb mode and re-applied after it.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• [Bug] Wrong RGB color and speed set after cycling through all RGB effects #493

Checklist

• [X ] My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).