[Coastrnaut]

Two cases left a sampler_randNN sampler used by a preset shader undeclared, so HLSL parsing failed and the default shader was used:

• No matching texture. If no texture file was available, or none matched the prefix of a name like rand00_smalltiled, GetRandomTexture() returned an empty descriptor, which declares nothing. A missing regular texture uses the placeholder texture instead, so the shader still compiles. This does the same for random textures. It matters most for apps that don't ship a texture pack.
• Same slot, different names. The texture of a random slot is shared between the warp and composite shader, but the second shader reused the first one's descriptor including its sampler name. If the shaders use different names for the same slot, e.g. sampler_rand00 and sampler_rand00_smalltiled, the second name was never declared. The texture is still shared, but the descriptor now uses the current shader's sampler name.

Results

• Without any texture files, presets falling back to the default shader dropped from 253 to 146 with this PR alone; most of the remaining ones are fixed by the other open shader PRs.
• With the preset folders as texture search path, 5 more presets compile both of their shaders, e.g. amandio c, flexi - Op illusions - random1 and Flexi - mindblob [where it's at now] gno sth air fetch.
• For all other presets, shader compile/link results and rendered frames are identical to master.
• All unit tests pass. TextureManager needs a GL context, so this is covered by the preset run.

Note: #1042 changes the same lines in MilkdropShader::LoadTexturesAndCompile() where the slot descriptor is reused, so whichever is merged second needs a small manual merge: create the descriptor with the current shader's sampler name, then apply PrefixSampler().

How I tested it

I loaded all 14,536 presets from cream-of-the-crop, en-d, milkdrop-original and projectm-classic one after another (Windows, RTX A6000, GL 3.3 core and GLES contexts), both with the preset folders as texture search path and without any textures, rendered frames with fixed audio and frame times, and recorded shader compile/link results, failed loads, logged shader fallbacks and frame hashes. For the pixel comparison both builds had their random seeds fixed (a local patch only, identical on both sides). The presets this PR fixes are excluded from the pixel comparison, since loading them changes shared state (e.g. the expression evaluator's random generator) for every preset after them.

Retested on current master (2026-11-12)

• Unit tests: 164/164 libprojectM and 98/98 playlist tests pass.
• Presets: I loaded all 14,536 presets from cream-of-the-crop, en-d, milkdrop-original and projectm-classic one after another with this branch and with master (GL 3.3 core, RTX A6000, preset folders as texture search path, 30 frames each with fixed audio, frame times and random seeds; the seeds are a local patch only, identical on both sides). The only compile/link changes are 4 presets whose shaders now compile (with textures available); no preset gets worse. All frames before the first of them are identical to master; later ones differ because each preset continues from the state the previous one left behind.
• Together with my other PRs: All my open PRs (Fix calling convention of glGetStringi pointer in GLProbe #1059 to Build for Windows Store (UWP) without app-side workarounds #1065, Return int from the NaN-safe multiplication of two ints #1067 to Fix shaders that declare or name samplers through macros #1072, Build Apple frameworks for iOS, tvOS, watchOS and visionOS, and support EAGL contexts #1075, Don't shift the blur textures by Direct3D's half-pixel offset #1078 and Render with 10 bits per color channel if the target framebuffer has more than 8 #1079) merge with each other and with master without conflicts. Merged together (all except Render with 10 bits per color channel if the target framebuffer has more than 8 #1079, which I opened after this run), they build on desktop GL and GLES, all unit tests pass, and the corpus gives 0 failed loads, 0 shader compile/link failures and 0 GL errors in an OpenGL ES context, with and without textures.