[Fix, Test] (TextureFormatProcessor): store the desktop-only low-bit formats without a driver requantization

MobileGL/MG_Test/Texture/TextureTest.cpp

@@ -3197,11 +3197,19 @@ TEST_F(TextureTest, NormalizeLegacySizedFormatsMapToCanonicalShadowLayouts) {
         GLenum type;
     };
     const Case cases[] = {
-        // Legacy <=8-bit-per-channel formats store as UNorm8 component arrays.
+        // Legacy <=8-bit-per-channel DESKTOP-ONLY formats store as UNorm8 component arrays, in the
-        {GL_R3_G3_B2, GL_RGB565, GL_RGB, GL_UNSIGNED_BYTE},
+        // 8-bit-per-channel ES format that layout already is. Storing them in the narrower
-        {GL_RGB4, GL_RGB565, GL_RGB, GL_UNSIGNED_BYTE},
+        // GL_RGB565/GL_RGBA4 they nominally fit in made the driver requantize the shadow bytes on
-        {GL_RGB5, GL_RGB565, GL_RGB, GL_UNSIGNED_BYTE},
+        // every upload, which is not lossless: 5-bit 2 -> UNorm8 16 -> 16/255*31 = 1.945, which a
-        {GL_RGBA2, GL_RGBA4, GL_RGBA, GL_UNSIGNED_BYTE},
+        // truncating driver reads back as 1 (KHR-GL43.copy_image rgb4->rgb4, 12/12 failing on Mali).
+        {GL_R3_G3_B2, GL_RGB8, GL_RGB, GL_UNSIGNED_BYTE},
+        {GL_RGB4, GL_RGB8, GL_RGB, GL_UNSIGNED_BYTE},
+        {GL_RGB5, GL_RGB8, GL_RGB, GL_UNSIGNED_BYTE},
+        {GL_RGBA2, GL_RGBA8, GL_RGBA, GL_UNSIGNED_BYTE},
+        // The two that are ES formats in their own right keep their native storage: an application
+        // that asks for GL_RGBA4 or GL_RGB5_A1 is asking for the smaller image, and the same
+        // normalization also picks the storage for glRenderbufferStorage, where those two are
+        // ordinary ES render targets rather than a desktop-compatibility shim.
         {GL_RGBA4, GL_RGBA4, GL_RGBA, GL_UNSIGNED_BYTE},
         {GL_RGB5_A1, GL_RGB5_A1, GL_RGBA, GL_UNSIGNED_BYTE},
         // 10/12-bit channels store as UNorm16 component arrays.

MobileGL/MG_Util/Texture/TextureFormatProcessor.cpp

@@ -270,10 +270,30 @@ namespace MobileGL::MG_Util::TextureFormatProcessor {
             // per-channel precision (extra precision stays inside the CTS comparison epsilon, which is
             // derived from the requested format's bit widths). The upload (format, type) below matches
             // the canonical shadow layout in PixelStoreProcessor (UNorm8 / UNorm16 component arrays).
+            //
+            // The <=8-bit ones land on the 8-bit-per-channel storage that layout ALREADY is, rather
+            // than on the narrower GL_RGB565/GL_RGBA4 they nominally fit in. Storing them narrower
+            // made the driver requantize the UNorm8 shadow bytes on every upload, and that step is
+            // exact only by luck: 5-bit value 2 encodes as UNorm8 16, and 16/255*31 = 1.945 sits
+            // astride the 5-bit boundary, so a driver that truncates hands back 1 (all twelve
+            // KHR-GL43.copy_image.functional rgb4->rgb4 cases fail on Mali, at verify()'s FIRST
+            // check - a plain glTexImage/glGetTexImage round trip with no copy involved). The
+            // 8-bit store removes the requantization entirely; the client word round-trips
+            // exactly, because encoding an n-bit field to UNorm8 with rounding and back is the
+            // identity for every n <= 8. It is also what DirectVulkan has always done with them
+            // (VkTextureManager::ResolveTextureFormatInfo resolves all six legacy low-bit formats
+            // to R8G8B8A8_UNORM), so the two backends now agree here.
+            //
+            // Only the DESKTOP-ONLY formats move. GL_RGBA4 and GL_RGB5_A1 are ES formats an
+            // application can legitimately ask for - the same normalization picks the storage for
+            // glRenderbufferStorage - so widening them would be a memory decision, not a
+            // correctness one. Nothing about the REPORTED precision moves either way:
+            // GL_TEXTURE_*_SIZE and glGetInternalformativ answer from TextureMetrics, keyed on the
+            // requested format, not on the ES storage.
             case GL_R3_G3_B2:
             case GL_RGB4:
             case GL_RGB5:
-                *outInternalFormat = GL_RGB565;
+                *outInternalFormat = GL_RGB8;
                 break;
             case GL_RGB10:
             case GL_RGB12:
@@ -283,7 +303,7 @@ namespace MobileGL::MG_Util::TextureFormatProcessor {
                                          : GL_RGB16;
                 break;
             case GL_RGBA2:
-                *outInternalFormat = GL_RGBA4;
+                *outInternalFormat = GL_RGBA8;
                 break;
             case GL_RGBA12:
                 *outInternalFormat =