feat: enable OrigTexture in glsl shaders, fix shader UI desync

[DrFlarp]

Implement OrigTexture so all shaders in the chain can access the unaltered core output image.

Also addresses #678 with a proposed fix: split Shader into two objects, let's call them ShaderProgram and ShaderPass

ShaderProgram: contains only values directly pertaining to an individual .glsl compilation unit

• shader program handle
• uniform locations
• pragma uniform locations

(This allows us to reuse all of this information when we run overlay.glsl up to 3 times in our pipeline, each time with different configs)

ShaderPass: contains everything else previously held in Shader

• alpha has been added as a property to this struct (removed from runShaderPass call)

Other related changes/fixes:

• a bunch of shader init code has been moved from PLAT_updateShader to a new function init_shader_program that sets up a ShaderProgram object. This new function is now also called by PLAT_initShaders to set up the .system shaders.
• g_shader_default et. al have been removed and replaced by ShaderPrograms, they still serve the same purpose but are now bundled with their uniform locations
• RunShaderPass call has been changed, I'm not fully satisfied with what I've come up and would like one more iteration on this. (in particular, the way srcw/h, texw/h is also a part of ShaderPass but not used by the function) (edit: better now!)
• RunShaderPass: filter is now called next_filter for clarity
• Checking for null shader prorgram has been moved from PLAT_GL_Swap to RunShaderPass
• bugfix: OrigInputSize now always properly provides the size of the core's framebuffer (instead of srcw/h which might have been scaled if srctype is relative)
• OrigTextureSize also added (matches OrigInputSize for now)
• load_shader_from_file now takes a full filepath (the caller has already concatenated it for us)

The only behavioral changes should be the availability of OrigTexture/OrigTextureSize and the fix for OrigInputSize which only affects a very specific setup of lcd1x (the included presets real-gameboy and real-gba still work the same). As far as my testing goes, all other shaders should work exactly as before.

[DrFlarp]

huh, build-linux is failing on building settings.elf. nextui and minarch seem to build fine however

Edit: github ci runners seem to be janking us out

[DrFlarp]

Ok, it took a few tries but I think it's ready to go :)