RENDERER: Fix animation interpolation at cycle boundaries for alias models

[qqshka]

Previously, animation interpolation was broken at loop boundaries (e.g.
frame 7 -> frame 0) for both MDL and MD3 models. This caused visible
snapping/popping when an animation cycled back to the beginning.

Root causes:

1. R_AliasModelDeterminePoses set lerp=1 for non-sequential frame pairs,
   disabling interpolation at cycle boundaries.
2. R_AliasModelPrepare checked that frame2 == expected_next_frame,
   rejecting legitimate cross-boundary lerp pairs.
3. Classic GL shaders used precomputed direction vectors (nextFrame -
   currentFrame) baked into VBO at load time. At cycle boundaries these
   vectors point to the wrong target frame, producing garbage geometry.

Classic GL fix (macOS, RENDERER_MODERN_OPENGL=OFF):

• Remove the non-sequential frame guard in R_AliasModelDeterminePoses
  so lerpfrac = r_framelerp for all frame pairs.
• Remove the expected_frame check in R_AliasModelPrepare.
• Change all classic GL alias model vertex shaders from the precomputed-
  direction formula (pos + dir * t) to mix(pos1, pos2, t), where
  gl_MultiTexCoord1 is rebound per draw call to carry pose2 positions
  instead of baked direction vectors.
• Add GLC_BindPose2TextureCoord[_MD3]() helpers that override the
  VAO's texcoord1 binding after each state setup call (which resets the
  VAO), pointing it at the VBO region for the second pose.
• When pose2 precedes pose1 in the VBO (cycle boundary where frame2 <
  frame1), swap the two poses and invert lerpfrac to ensure the VBO
  offset (pose2Vert - firstVert) is always non-negative. Swapping is
  mathematically equivalent: mix(a, b, t) == mix(b, a, 1-t).
• Pass pose2 to GLC_DrawPowerupShell_Program and
  GLC_DrawAliasModelShadowDrawCall_Program (previously only received
  pose1), and apply the same rebind + swap logic there.
• Inline GLC_DrawCachedAliasOutlineFrame at its call site so that
  GLC_BindPose2TextureCoord can be inserted between the state setup
  and GL_DrawArrays; remove the now-unused function.

Modern GL fix (Linux/Windows, RENDERER_MODERN_OPENGL=ON):

• Add pose1Base / pose2Base fields to the per-instance uniform block
  so the vertex shader can locate the correct second-pose data.
• Create a per-model SSBO (r_buffer_aliasmodel_vertex_ssbo) containing
  the full vertex array as raw floats (same data as the VBO).
• In the vertex shader, use gl_VertexID to index into the SSBO and
  read pose2 position directly, replacing the pos + vboDirection * t
  formula with mix(vboPosition, pose2Position, t).
• Bind the SSBO in GLM_PrepareAliasModelBatches before the draw.

Affected files:
src/r_aliasmodel.c - lerp guards
src/r_aliasmodel_mesh.c - SSBO creation (modern GL)
src/glc_aliasmodel.c - pose2 rebind helpers, swap logic (MDL)
src/glc_md3.c - pose2 rebind helpers, swap logic (MD3)
src/glm_aliasmodel.c - pose1Base/pose2Base uniforms, SSBO bind
src/glsl/common.glsl - pose1Base/pose2Base in AliasModel struct
src/glsl/constants.glsl - EZQ_GL_BINDINGPOINT_ALIASMODEL_SSBO = 6
src/glsl/draw_aliasmodel.vertex.glsl - SSBO read + mix() (modern GL)
src/glsl/glc/glc_aliasmodel_std.vertex.glsl - mix() formula (classic)
src/glsl/glc/glc_aliasmodel_shadow.vertex.glsl - mix() formula (classic)
src/glsl/glc/glc_aliasmodel_shell.vertex.glsl - mix() formula (classic)

[ciscon]

as of now i get a ton of artifacts

[qqshka]

as of now i get a ton of artifacts

yeah, as I mention in description, you probably using vid_rendrer 1, could not test it. Anyway, PR a draft for a reason.