[Feat] (MG_Impl, MG_State, MG_Util): attach a buffer texture to a range of its buffer

glTexBufferRange, glTextureBuffer and glTextureBufferRange were all stubs, so a
buffer texture could only ever be attached through glTexBuffer -- by binding, and
always to the whole buffer.

Give the buffer texture the window it is supposed to address. The non-range forms
record it as offset 0 with a whole-buffer sentinel rather than the size the buffer
happens to have, so a later respecify keeps being followed instead of freezing the
texture at yesterday's size. All four entry points now share one attach path,
differing only in how they name the texture: by binding for the target forms, by
name for the DSA ones.

Both backends honour the window: DirectVulkan offsets and clamps the buffer view,
DirectGLES uses glTexBufferRange when the texture names a sub-range and keeps
plain glTexBuffer for the whole-buffer case, which also works on a driver without
the range entry point.

GL_TEXTURE_BUFFER_OFFSET_ALIGNMENT reported 0 with a comment explaining that the
range entry points were stubbed. It now reports what the device actually requires
-- minTexelBufferOffsetAlignment on Vulkan, the driver's own value on GLES -- and
the range entry points enforce it. Zero was never a legal answer; the minimum is
1, and an application that trusted it would have built unaligned offsets.

MobileGL/MG_Backend/BackendObject.h

@@ -315,6 +315,8 @@ namespace MobileGL {
             Int MaxComputeWorkGroupInvocations = 128;
             Int MaxShaderStorageBufferBindings = 8;
             Int MaxTextureBufferSize = 65536;
+            // GL_TEXTURE_BUFFER_OFFSET_ALIGNMENT; 1 means the offset is unconstrained.
+            Int TextureBufferOffsetAlignment = 1;
             Int MaxUniformBufferBindings = 24;
             Int MaxUniformBlockSize = 16384;
             Int MaxImageUnits = 8;