Add test & fix for unusual pitch in `surface.premul_alpha()`

[MyreMylar]

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fixes #2750

From what I understand, these unusual, non-pixel aligned, surface pitches won't practically come up on any modern desktop systems (possibly no modern systems) so I prioritised fixing it for the least performance sensitive versions of this function - sse2 & the non-SIMD fallback. These are also the versions I originally wrote so I had a better idea of how they were supposed to work.

The basic fix is to add in the standard 'skip' value that is used in all the blitters to handle pitch issues between two different surfaces - usually these are pixel aligned. In the SSE2 case, to deal with the .5 of a pixel overlap in the pitch case we have to cast the skip value down to Uint8 to get to 'channel', or single byte, level of pointer incrementing as we only want to skip 2 channels worth of a pixel (2 bytes) rather than a whole pixel (4 bytes).

I think that makes sense anyway.

This probably needs feedback from @itzpr3d4t0r and @Starbuck5 to see if they think what I've changed makes sense and if we need to do anything else here.

[ankith26]

IG the same approach should probably be taken on all simd surface manipulations not just premul

[coderabbitai[bot]]

📝 Walkthrough

Walkthrough

Adds pitch-aware row advancement to premultiply-alpha routines and tight-packing guards for AVX2. Updates SSE2 and non-SIMD paths to compute per-row skips from pitch and advance pointers accordingly. Adds a test verifying premultiplication correctness on surfaces with non-standard pitch.

Changes

Cohort / File(s)	Summary
Core premul alpha path src_c/alphablit.c	AVX2 fast path now requires tight packing (pitch == width × bpp). Non-SIMD path computes src/dst row skips from pitch and advances pointers after each row to respect padding. No API changes.
SIMD SSE2 premul src_c/simd_blitters_sse2.c	Introduces srcskip/dstskip based on pitch; advances srcp/dstp per row using byte-wise arithmetic. Per-pixel logic unchanged.
Tests for odd pitches test/surface_test.py	Adds test creating surfaces from buffers with custom pitch (e.g., 10 bytes for 2px width) and asserts correct premul_alpha results across all pixels.

Sequence Diagram(s)

sequenceDiagram
  autonumber
  participant Py as Python caller
  participant Premul as premul_alpha()
  participant AVX2 as AVX2 path
  participant SSE2 as SSE2 path
  participant Scalar as Non-SIMD path

  Py->>Premul: request premultiply (src, dst)
  Premul->>Premul: Check format, dims, pitch
  alt Tight-packed and AVX2 available
    Premul->>AVX2: process with AVX2
    note right of AVX2: Guard: pitch == width*bpp
    AVX2-->>Premul: done
  else AVX2 not used
    alt SSE2 available
      Premul->>SSE2: process row by row
      note right of SSE2: Compute srcskip/dstskip = pitch - width*bpp<br/>Advance pointers by skips each row
      SSE2-->>Premul: done
    else
      Premul->>Scalar: process row by row
      note right of Scalar: Compute srcskip/dstskip and advance each row
      Scalar-->>Premul: done
    end
  end
  Premul-->>Py: result surface

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Assessment against linked issues

Objective	Addressed	Explanation
Respect non-standard surface pitches in premul_alpha, including unaligned pitch (e.g., 10 bytes for 2px width) [#2750]	✅

Assessment against linked issues: Out-of-scope changes

None found.

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