GS: Unify HW texture caches

[TellowKrinkle]

Description of Changes

• Converts a number enums to enum classes, with arguments switching to typed values instead of int
• Stores a known enum in the format field of textures, removing issues where e.g. no one outside of GSDeviceOGL could actually understand what the format of a GSTextureOGL meant even though they could read it. Now anyone can ask a GSTexture* for its format and understand what it is.
• Adds a DownloadTexture and an optional DownloadTextureConvert method to GSDevice, which downloads a texture and maps it to CPU memory
• Uses the above changes to add generic, shared versions of all the virtual GSTextureCache methods to GSTextureCache and get rid of the separate subclasses per subclass of GSDevice
• Renames convert shaders to not be ps_convert## because it's impossible to figure out what's what when only looking at the list of shaders

Rationale behind Changes

Reduce per-renderer code so that adding a new renderer doesn't add as much maintenance cost

Suggested Testing Steps

Make sure renderers still work

[JordanTheToaster]

Tested with my selection of games on both DX11 and OpenGL and both functioned correctly with no issues I could observe.

[refractionpcsx2]

Ran it through a bunch of gsdumps on DX and OGL, looks good to me :)

[lightningterror]

Ran through some dumps, everything seemed fine.

[lightningterror]

Hmm, we have 6 total free vectors, this can be optimized.